kbuild: prefix $(srctree)/ to some included Makefiles

VPATH is used in Kbuild to make pattern rules search for prerequisites
in both $(objtree) and $(srctree). Some of *.c, *.S files are not real
sources, but generated by tools such as flex, bison, perl.

In contrast, I doubt the benefit of --include-dir=$(abs_srctree) because
it is always clear which Makefiles are real sources, and which are not.

So, my hope is to add $(srctree)/ prefix to all check-in Makefiles,
then remove --include-dir=$(abs_srctree) flag in the future.

I am touching only some Kbuild core parts for now. Treewide fixes will
be needed to achieve this goal.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index 066beff..df57e25 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -41,10 +41,10 @@
 __modpost:
 
 include include/config/auto.conf
-include scripts/Kbuild.include
+include $(srctree)/scripts/Kbuild.include
 
 # for ld_flags
-include scripts/Makefile.lib
+include $(srctree)/scripts/Makefile.lib
 
 MODPOST = scripts/mod/modpost								\
 	$(if $(CONFIG_MODVERSIONS),-m)							\