kbuild: collect shorthands into scripts/Kbuild.include

The shorthand "clean" is defined in both the top Makefile and
scripts/Makefile.clean.  Likewise, the "hdr-inst" is defined in
both the top Makefile and scripts/Makefile.headersinst.

To reduce code duplication, this commit collects them into
scripts/Kbuild.include like the "build" and "modbuiltin" shorthands.
It requires scripts/Makefile.clean to include scripts/Kbuild.include,
but its impact on the performance of "make clean" should be
negligible.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/scripts/Makefile.clean b/scripts/Makefile.clean
index b1c668d..0aa91a0 100644
--- a/scripts/Makefile.clean
+++ b/scripts/Makefile.clean
@@ -7,10 +7,7 @@
 PHONY := __clean
 __clean:
 
-# Shorthand for $(Q)$(MAKE) scripts/Makefile.clean obj=dir
-# Usage:
-# $(Q)$(MAKE) $(clean)=dir
-clean := -f $(srctree)/scripts/Makefile.clean obj
+include scripts/Kbuild.include
 
 # The filename Kbuild has precedence over Makefile
 kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))