// -*- mode:doc; -*-
// vim: set syntax=asciidoc:

=== Infrastructure for packages with specific build systems

By 'packages with specific build systems' we mean all the packages
whose build system is not one of the standard ones, such as
'autotools' or 'CMake'. This typically includes packages whose build
system is based on hand-written Makefiles or shell scripts.

[[generic-package-tutorial]]

==== +generic-package+ tutorial

------------------------------
01: ################################################################################
02: #
03: # libfoo
04: #
05: ################################################################################
06:
07: LIBFOO_VERSION = 1.0
08: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
09: LIBFOO_SITE = http://www.foosoftware.org/download
10: LIBFOO_LICENSE = GPL-3.0+
11: LIBFOO_LICENSE_FILES = COPYING
12: LIBFOO_INSTALL_STAGING = YES
13: LIBFOO_CONFIG_SCRIPTS = libfoo-config
14: LIBFOO_DEPENDENCIES = host-libaaa libbbb
15:
16: define LIBFOO_BUILD_CMDS
17:	$(MAKE) $(TARGET_CONFIGURE_OPTS) -C $(@D) all
18: endef
19:
20: define LIBFOO_INSTALL_STAGING_CMDS
21:	$(INSTALL) -D -m 0755 $(@D)/libfoo.a $(STAGING_DIR)/usr/lib/libfoo.a
22:	$(INSTALL) -D -m 0644 $(@D)/foo.h $(STAGING_DIR)/usr/include/foo.h
23:	$(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(STAGING_DIR)/usr/lib
24: endef
25:
26: define LIBFOO_INSTALL_TARGET_CMDS
27:	$(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(TARGET_DIR)/usr/lib
28:	$(INSTALL) -d -m 0755 $(TARGET_DIR)/etc/foo.d
29: endef
30:
31: define LIBFOO_USERS
32:	foo -1 libfoo -1 * - - - LibFoo daemon
33: endef
34:
35: define LIBFOO_DEVICES
36:	/dev/foo  c  666  0  0	42  0  -  -  -
37: endef
38:
39: define LIBFOO_PERMISSIONS
40:	/bin/foo  f  4755  foo  libfoo	 -  -  -  -  -
41: endef
42:
43: $(eval $(generic-package))
--------------------------------

The Makefile begins on line 7 to 11 with metadata information: the
version of the package (+LIBFOO_VERSION+), the name of the
tarball containing the package (+LIBFOO_SOURCE+) (xz-ed tarball recommended)
the Internet location at which the tarball can be downloaded from
(+LIBFOO_SITE+), the license (+LIBFOO_LICENSE+) and file with the
license text (+LIBFOO_LICENSE_FILES+). All variables must start with
the same prefix, +LIBFOO_+ in this case. This prefix is always the
uppercased version of the package name (see below to understand where
the package name is defined).

On line 12, we specify that this package wants to install something to
the staging space. This is often needed for libraries, since they must
install header files and other development files in the staging space.
This will ensure that the commands listed in the
+LIBFOO_INSTALL_STAGING_CMDS+ variable will be executed.

On line 13, we specify that there is some fixing to be done to some
of the 'libfoo-config' files that were installed during
+LIBFOO_INSTALL_STAGING_CMDS+ phase.
These *-config files are executable shell script files that are
located in '$(STAGING_DIR)/usr/bin' directory and are executed
by other 3rd party packages to find out the location and the linking
flags of this particular package.

The problem is that all these *-config files by default give wrong,
host system linking flags that are unsuitable for cross-compiling.

For example:	'-I/usr/include' instead of '-I$(STAGING_DIR)/usr/include'
or:		'-L/usr/lib' instead of '-L$(STAGING_DIR)/usr/lib'

So some sed magic is done to these scripts to make them give correct
flags.
The argument to be given to +LIBFOO_CONFIG_SCRIPTS+ is the file name(s)
of the shell script(s) needing fixing. All these names are relative to
'$(STAGING_DIR)/usr/bin' and if needed multiple names can be given.

In addition, the scripts listed in +LIBFOO_CONFIG_SCRIPTS+ are removed
from +$(TARGET_DIR)/usr/bin+, since they are not needed on the target.

.Config script: 'divine' package
================================
Package divine installs shell script '$(STAGING_DIR)/usr/bin/divine-config'.

So its fixup would be:

--------------------------------
DIVINE_CONFIG_SCRIPTS = divine-config
--------------------------------
================================

.Config script: 'imagemagick' package:
================================
Package imagemagick installs the following scripts:
'$(STAGING_DIR)/usr/bin/{Magick,Magick++,MagickCore,MagickWand,Wand}-config'

So it's fixup would be:

--------------------------------
IMAGEMAGICK_CONFIG_SCRIPTS = \
   Magick-config Magick++-config \
   MagickCore-config MagickWand-config Wand-config
--------------------------------
================================

On line 14, we specify the list of dependencies this package relies
on. These dependencies are listed in terms of lower-case package names,
which can be packages for the target (without the +host-+
prefix) or packages for the host (with the +host-+) prefix).
Buildroot will ensure that all these packages are built and installed
'before' the current package starts its configuration.

The rest of the Makefile, lines 16..29, defines what should be done
at the different steps of the package configuration, compilation and
installation.
+LIBFOO_BUILD_CMDS+ tells what steps should be performed to
build the package. +LIBFOO_INSTALL_STAGING_CMDS+ tells what
steps should be performed to install the package in the staging space.
+LIBFOO_INSTALL_TARGET_CMDS+ tells what steps should be
performed to install the package in the target space.

All these steps rely on the +$(@D)+ variable, which
contains the directory where the source code of the package has been
extracted.

On lines 31..43, we define a user that is used by this package (e.g.
to run a daemon as non-root) (+LIBFOO_USERS+).

On line 35..37, we define a device-node file used by this package
(+LIBFOO_DEVICES+).

On line 39..41, we define the permissions to set to specific files
installed by this package (+LIBFOO_PERMISSIONS+).

Finally, on line 43, we call the +generic-package+ function, which
generates, according to the variables defined previously, all the
Makefile code necessary to make your package working.

[[generic-package-reference]]

==== +generic-package+ reference

There are two variants of the generic target. The +generic-package+ macro is
used for packages to be cross-compiled for the target. The
+host-generic-package+ macro is used for host packages, natively compiled
for the host. It is possible to call both of them in a single +.mk+
file: once to create the rules to generate a target
package and once to create the rules to generate a host package:

----------------------
$(eval $(generic-package))
$(eval $(host-generic-package))
----------------------

This might be useful if the compilation of the target package requires
some tools to be installed on the host. If the package name is
+libfoo+, then the name of the package for the target is also
+libfoo+, while the name of the package for the host is
+host-libfoo+. These names should be used in the DEPENDENCIES
variables of other packages, if they depend on +libfoo+ or
+host-libfoo+.

The call to the +generic-package+ and/or +host-generic-package+ macro *must* be
at the end of the +.mk+ file, after all variable definitions.

For the target package, the +generic-package+ uses the variables defined by
the .mk file and prefixed by the uppercased package name:
+LIBFOO_*+. +host-generic-package+ uses the +HOST_LIBFOO_*+ variables. For
'some' variables, if the +HOST_LIBFOO_+ prefixed variable doesn't
exist, the package infrastructure uses the corresponding variable
prefixed by +LIBFOO_+. This is done for variables that are likely to
have the same value for both the target and host packages. See below
for details.

The list of variables that can be set in a +.mk+ file to give metadata
information is (assuming the package name is +libfoo+) :

* +LIBFOO_VERSION+, mandatory, must contain the version of the
  package. Note that if +HOST_LIBFOO_VERSION+ doesn't exist, it is
  assumed to be the same as +LIBFOO_VERSION+. It can also be a
  revision number, branch or tag for packages that are fetched
  directly from their revision control system. +
  Examples: +
    +LIBFOO_VERSION = 0.1.2+ +
    +LIBFOO_VERSION = cb9d6aa9429e838f0e54faa3d455bcbab5eef057+ +
    +LIBFOO_VERSION = stable+

* +LIBFOO_SOURCE+ may contain the name of the tarball of the package,
  which Buildroot will use to download the tarball from
  +LIBFOO_SITE+. If +HOST_LIBFOO_SOURCE+ is not specified, it defaults
  to +LIBFOO_SOURCE+. If none are specified, then the value is assumed
  to be +libfoo-$(LIBFOO_VERSION).tar.gz+. +
  Example: +LIBFOO_SOURCE = foobar-$(LIBFOO_VERSION).tar.bz2+

* +LIBFOO_PATCH+ may contain a space-separated list of patch file
  names, that Buildroot will download and apply to the package source
  code. If an entry contains +://+, then Buildroot will assume it is a
  full URL and download the patch from this location. Otherwise,
  Buildroot will assume that the patch should be downloaded from
  +LIBFOO_SITE+. If +HOST_LIBFOO_PATCH+ is not specified, it defaults
  to +LIBFOO_PATCH+. Note that patches that are included in Buildroot
  itself use a different mechanism: all files of the form
  +*.patch+ present in the package directory inside
  Buildroot will be applied to the package after extraction (see
  xref:patch-policy[patching a package]). Finally, patches listed in
  the +LIBFOO_PATCH+ variable are applied _before_ the patches stored
  in the Buildroot package directory.

* +LIBFOO_SITE+ provides the location of the package, which can be a
  URL or a local filesystem path. HTTP, FTP and SCP are supported URL
  types for retrieving package tarballs. In these cases don't include a
  trailing slash: it will be added by Buildroot between the directory
  and the filename as appropriate. Git, Subversion, Mercurial,
  and Bazaar are supported URL types for retrieving packages directly
  from source code management systems. There is a helper function to make
  it easier to download source tarballs from GitHub (refer to
  xref:github-download-url[] for details). A filesystem path may be used
  to specify either a tarball or a directory containing the package
  source code. See +LIBFOO_SITE_METHOD+ below for more details on how
  retrieval works. +
  Note that SCP URLs should be of the form
  +scp://[user@]host:filepath+, and that filepath is relative to the
  user's home directory, so you may want to prepend the path with a
  slash for absolute paths:
  +scp://[user@]host:/absolutepath+. +
  If +HOST_LIBFOO_SITE+ is not specified, it defaults to
  +LIBFOO_SITE+.
  Examples: +
    +LIBFOO_SITE=http://www.libfoosoftware.org/libfoo+ +
    +LIBFOO_SITE=http://svn.xiph.org/trunk/Tremor+ +
    +LIBFOO_SITE=/opt/software/libfoo.tar.gz+ +
    +LIBFOO_SITE=$(TOPDIR)/../src/libfoo+

* +LIBFOO_DL_OPTS+ is a space-separated list of additional options to
  pass to the downloader. Useful for retrieving documents with
  server-side checking for user logins and passwords, or to use a proxy.
  All download methods valid for +LIBFOO_SITE_METHOD+ are supported;
  valid options depend on the download method (consult the man page
  for the respective download utilities).

* +LIBFOO_EXTRA_DOWNLOADS+ is a space-separated list of additional
  files that Buildroot should download. If an entry contains +://+
  then Buildroot will assume it is a complete URL and will download
  the file using this URL. Otherwise, Buildroot will assume the file
  to be downloaded is located at +LIBFOO_SITE+. Buildroot will not do
  anything with those additional files, except download them: it will
  be up to the package recipe to use them from +$(DL_DIR)+.

* +LIBFOO_SITE_METHOD+ determines the method used to fetch or copy the
  package source code. In many cases, Buildroot guesses the method
  from the contents of +LIBFOO_SITE+ and setting +LIBFOO_SITE_METHOD+
  is unnecessary. When +HOST_LIBFOO_SITE_METHOD+ is not specified, it
  defaults to the value of +LIBFOO_SITE_METHOD+. +
  The possible values of +LIBFOO_SITE_METHOD+ are:
  ** +wget+ for normal FTP/HTTP downloads of tarballs. Used by
     default when +LIBFOO_SITE+ begins with +http://+, +https://+ or
     +ftp://+.
  ** +scp+ for downloads of tarballs over SSH with scp. Used by
     default when +LIBFOO_SITE+ begins with +scp://+.
  ** +svn+ for retrieving source code from a Subversion repository.
     Used by default when +LIBFOO_SITE+ begins with +svn://+. When a
     +http://+ Subversion repository URL is specified in
     +LIBFOO_SITE+, one 'must' specify +LIBFOO_SITE_METHOD=svn+.
     Buildroot performs a checkout which is preserved as a tarball in
     the download cache; subsequent builds use the tarball instead of
     performing another checkout.
  ** +cvs+ for retrieving source code from a CVS repository.
     Used by default when +LIBFOO_SITE+ begins with +cvs://+.
     The downloaded source code is cached as with the +svn+ method.
     Anonymous pserver mode is assumed otherwise explicitly defined
     on +LIBFOO_SITE+. Both
     +LIBFOO_SITE=cvs://libfoo.net:/cvsroot/libfoo+ and
     +LIBFOO_SITE=cvs://:ext:libfoo.net:/cvsroot/libfoo+
     are accepted, on the former anonymous pserver access mode is
     assumed.
     +LIBFOO_SITE+ 'must' contain the source URL as well as the remote
     repository directory. The module is the package name.
     +LIBFOO_VERSION+ is 'mandatory' and 'must' be a tag, a branch, or
     a date (e.g. "2014-10-20", "2014-10-20 13:45", "2014-10-20
     13:45+01" see "man cvs" for further details).
  ** +git+ for retrieving source code from a Git repository. Used by
     default when +LIBFOO_SITE+ begins with +git://+. The downloaded
     source code is cached as with the +svn+
     method.
  ** +hg+ for retrieving source code from a Mercurial repository. One
     'must' specify +LIBFOO_SITE_METHOD=hg+ when +LIBFOO_SITE+
     contains a Mercurial repository URL. The downloaded source code
     is cached as with the +svn+ method.
  ** +bzr+ for retrieving source code from a Bazaar repository. Used
     by default when +LIBFOO_SITE+ begins with +bzr://+. The
     downloaded source code is cached as with the +svn+ method.
  ** +file+ for a local tarball. One should use this when
     +LIBFOO_SITE+ specifies a package tarball as a local filename.
     Useful for software that isn't available publicly or in version
     control.
  ** +local+ for a local source code directory. One should use this
     when +LIBFOO_SITE+ specifies a local directory path containing
     the package source code. Buildroot copies the contents of the
     source directory into the package's build directory. Note that
     for +local+ packages, no patches are applied. If you need to
     still patch the source code, use +LIBFOO_POST_RSYNC_HOOKS+, see
     xref:hooks-rsync[].

* +LIBFOO_GIT_SUBMODULES+ can be set to +YES+ to create an archive
  with the git submodules in the repository.  This is only available
  for packages downloaded with git (i.e. when
  +LIBFOO_SITE_METHOD=git+).  Note that we try not to use such git
  submodules when they contain bundled libraries, in which case we
  prefer to use those libraries from their own package.

* +LIBFOO_STRIP_COMPONENTS+ is the number of leading components
  (directories) that tar must strip from file names on extraction.
  The tarball for most packages has one leading component named
  "<pkg-name>-<pkg-version>", thus Buildroot passes
  --strip-components=1 to tar to remove it.
  For non-standard packages that don't have this component, or
  that have more than one leading component to strip, set this
  variable with the value to be passed to tar. Default: 1.

* +LIBFOO_EXCLUDES+ is a space-separated list of patterns to exclude
  when extracting the archive. Each item from that list is passed as
  a tar's +--exclude+ option. By default, empty.

* +LIBFOO_DEPENDENCIES+ lists the dependencies (in terms of package
  name) that are required for the current target package to
  compile. These dependencies are guaranteed to be compiled and
  installed before the configuration of the current package starts. In
  a similar way, +HOST_LIBFOO_DEPENDENCIES+ lists the dependencies for
  the current host package.

* +LIBFOO_PATCH_DEPENDENCIES+ lists the dependencies (in terms of
  package name) that are required for the current package to be
  patched. These dependencies are guaranteed to be extracted and
  patched before the current package is patched. In a similar way,
  +HOST_LIBFOO_PATCH_DEPENDENCIES+ lists the dependencies for the
  current host package.
  This is seldom used; usually, +LIBFOO_DEPENDENCIES+ is what you
  really want to use.

* +LIBFOO_PROVIDES+ lists all the virtual packages +libfoo+ is an
  implementation of. See xref:virtual-package-tutorial[].

* +LIBFOO_INSTALL_STAGING+ can be set to +YES+ or +NO+ (default). If
  set to +YES+, then the commands in the +LIBFOO_INSTALL_STAGING_CMDS+
  variables are executed to install the package into the staging
  directory.

* +LIBFOO_INSTALL_TARGET+ can be set to +YES+ (default) or +NO+. If
  set to +YES+, then the commands in the +LIBFOO_INSTALL_TARGET_CMDS+
  variables are executed to install the package into the target
  directory.

* +LIBFOO_INSTALL_IMAGES+ can be set to +YES+ or +NO+ (default). If
  set to +YES+, then the commands in the +LIBFOO_INSTALL_IMAGES_CMDS+
  variable are executed to install the package into the images
  directory.

* +LIBFOO_CONFIG_SCRIPTS+ lists the names of the files in
  '$(STAGING_DIR)/usr/bin' that need some special fixing to make them
  cross-compiling friendly. Multiple file names separated by space can
  be given and all are relative to '$(STAGING_DIR)/usr/bin'. The files
  listed in +LIBFOO_CONFIG_SCRIPTS+ are also removed from
  +$(TARGET_DIR)/usr/bin+ since they are not needed on the target.

* +LIBFOO_DEVICES+ lists the device files to be created by Buildroot
  when using the static device table. The syntax to use is the
  makedevs one. You can find some documentation for this syntax in the
  xref:makedev-syntax[]. This variable is optional.

* +LIBFOO_PERMISSIONS+ lists the changes of permissions to be done at
  the end of the build process. The syntax is once again the makedevs one.
  You can find some documentation for this syntax in the xref:makedev-syntax[].
  This variable is optional.

* +LIBFOO_USERS+ lists the users to create for this package, if it installs
  a program you want to run as a specific user (e.g. as a daemon, or as a
  cron-job). The syntax is similar in spirit to the makedevs one, and is
  described in the xref:makeuser-syntax[]. This variable is optional.

* +LIBFOO_LICENSE+ defines the license (or licenses) under which the package
  is released.
  This name will appear in the manifest file produced by +make legal-info+.
  If the license appears in https://spdx.org/licenses/[the SPDX License List],
  use the SPDX short identifier to make the manifest file uniform.
  Otherwise, describe the license in a precise and concise way, avoiding
  ambiguous names such as +BSD+ which actually name a family of licenses.
  This variable is optional. If it is not defined, +unknown+ will appear in
  the +license+ field of the manifest file for this package. +
  The expected format for this variable must comply with the following rules:
  ** If different parts of the package are released under different
  licenses, then +comma+ separate licenses (e.g. +`LIBFOO_LICENSE =
  GPL-2.0+, LGPL-2.1+`+). If there is clear distinction between which
  component is licensed under what license, then annotate the license
  with that component, between parenthesis (e.g. +`LIBFOO_LICENSE =
  GPL-2.0+ (programs), LGPL-2.1+ (libraries)`+).
  ** If the package is dual licensed, then separate licenses with the
  +or+ keyword (e.g. +`LIBFOO_LICENSE = AFL-2.1 or GPL-2.0+`+).

* +LIBFOO_LICENSE_FILES+ is a space-separated list of files in the package
  tarball that contain the license(s) under which the package is released.
  +make legal-info+ copies all of these files in the +legal-info+ directory.
  See xref:legal-info[] for more information.
  This variable is optional. If it is not defined, a warning will be produced
  to let you know, and +not saved+ will appear in the +license files+ field
  of the manifest file for this package.

* +LIBFOO_ACTUAL_SOURCE_TARBALL+ only applies to packages whose
  +LIBFOO_SITE+ / +LIBTOO_SOURCE+ pair points to an archive that does
  not actually contain source code, but binary code. This a very
  uncommon case, only known to apply to external toolchains which come
  already compiled, although theoretically it might apply to other
  packages. In such cases a separate tarball is usually available with
  the actual source code. Set +LIBFOO_ACTUAL_SOURCE_TARBALL+ to the
  name of the actual source code archive and Buildroot will download
  it and use it when you run +make legal-info+ to collect
  legally-relevant material.  Note this file will not be downloaded
  during regular builds nor by +make source+.

* +LIBFOO_ACTUAL_SOURCE_SITE+ provides the location of the actual
  source tarball. The default value is +LIBFOO_SITE+, so you don't
  need to set this variable if the binary and source archives are
  hosted on the same directory.  If +LIBFOO_ACTUAL_SOURCE_TARBALL+ is
  not set, it doesn't make sense to define
  +LIBFOO_ACTUAL_SOURCE_SITE+.

* +LIBFOO_REDISTRIBUTE+ can be set to +YES+ (default) or +NO+ to indicate if
  the package source code is allowed to be redistributed. Set it to +NO+ for
  non-opensource packages: Buildroot will not save the source code for this
  package when collecting the +legal-info+.

* +LIBFOO_FLAT_STACKSIZE+ defines the stack size of an application built into
  the FLAT binary format. The application stack size on the NOMMU architecture
  processors can't be enlarged at run time. The default stack size for the
  FLAT binary format is only 4k bytes. If the application consumes more stack,
  append the required number here.

The recommended way to define these variables is to use the following
syntax:

----------------------
LIBFOO_VERSION = 2.32
----------------------

Now, the variables that define what should be performed at the
different steps of the build process.

* +LIBFOO_EXTRACT_CMDS+ lists the actions to be performed to extract
  the package. This is generally not needed as tarballs are
  automatically handled by Buildroot. However, if the package uses a
  non-standard archive format, such as a ZIP or RAR file, or has a
  tarball with a non-standard organization, this variable allows to
  override the package infrastructure default behavior.

* +LIBFOO_CONFIGURE_CMDS+ lists the actions to be performed to
  configure the package before its compilation.

* +LIBFOO_BUILD_CMDS+ lists the actions to be performed to
  compile the package.

* +HOST_LIBFOO_INSTALL_CMDS+ lists the actions to be performed
  to install the package, when the package is a host package. The
  package must install its files to the directory given by
  +$(HOST_DIR)+. All files, including development files such as
  headers should be installed, since other packages might be compiled
  on top of this package.

* +LIBFOO_INSTALL_TARGET_CMDS+ lists the actions to be
  performed to install the package to the target directory, when the
  package is a target package. The package must install its files to
  the directory given by +$(TARGET_DIR)+. Only the files required for
  'execution' of the package have to be
  installed. Header files, static libraries and documentation will be
  removed again when the target filesystem is finalized.

* +LIBFOO_INSTALL_STAGING_CMDS+ lists the actions to be
  performed to install the package to the staging directory, when the
  package is a target package. The package must install its files to
  the directory given by +$(STAGING_DIR)+. All development files
  should be installed, since they might be needed to compile other
  packages.

* +LIBFOO_INSTALL_IMAGES_CMDS+ lists the actions to be performed to
  install the package to the images directory, when the package is a
  target package. The package must install its files to the directory
  given by +$(BINARIES_DIR)+. Only files that are binary images (aka
  images) that do not belong in the +TARGET_DIR+ but are necessary
  for booting the board should be placed here. For example, a package
  should utilize this step if it has binaries which would be similar
  to the kernel image, bootloader or root filesystem images.

* +LIBFOO_INSTALL_INIT_SYSV+ and +LIBFOO_INSTALL_INIT_SYSTEMD+ list the
  actions to install init scripts either for the systemV-like init systems
  (busybox, sysvinit, etc.) or for the systemd units. These commands
  will be run only when the relevant init system is installed (i.e. if
  systemd is selected as the init system in the configuration, only
  +LIBFOO_INSTALL_INIT_SYSTEMD+ will be run).

* +LIBFOO_HELP_CMDS+ lists the actions to print the package help, which
  is included to the main +make help+ output. These commands can print
  anything in any format.
  This is seldom used, as packages rarely have custom rules. *Do not use
  this variable*, unless you really know that you need to print help.

The preferred way to define these variables is:

----------------------
define LIBFOO_CONFIGURE_CMDS
	action 1
	action 2
	action 3
endef
----------------------

In the action definitions, you can use the following variables:

* +$(LIBFOO_PKGDIR)+ contains the path to the directory containing the
  +libfoo.mk+ and +Config.in+ files. This variable is useful when it is
  necessary to install a file bundled in Buildroot, like a runtime
  configuration file, a splashscreen image...

* +$(@D)+, which contains the directory in which the package source
  code has been uncompressed.

* +$(DL_DIR)+ contains the path to the directory where all the downloads made
  by Buildroot are stored.

* +$(TARGET_CC)+, +$(TARGET_LD)+, etc. to get the target
  cross-compilation utilities

* +$(TARGET_CROSS)+ to get the cross-compilation toolchain prefix

* Of course the +$(HOST_DIR)+, +$(STAGING_DIR)+ and +$(TARGET_DIR)+
  variables to install the packages properly.

Finally, you can also use hooks. See xref:hooks[] for more information.