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/Makefile b/Makefile
index a28bb37..93e6e2d2 100644
--- a/Makefile
+++ b/Makefile
@@ -339,14 +339,14 @@
 
 else # !mixed-build
 
-include scripts/Kbuild.include
+include $(srctree)/scripts/Kbuild.include
 
 # Read KERNELRELEASE from include/config/kernel.release (if it exists)
 KERNELRELEASE = $(shell cat include/config/kernel.release 2> /dev/null)
 KERNELVERSION = $(VERSION)$(if $(PATCHLEVEL),.$(PATCHLEVEL)$(if $(SUBLEVEL),.$(SUBLEVEL)))$(EXTRAVERSION)
 export VERSION PATCHLEVEL SUBLEVEL KERNELRELEASE KERNELVERSION
 
-include scripts/subarch.include
+include $(srctree)/scripts/subarch.include
 
 # Cross compiling and selecting different set of gcc/bin-utils
 # ---------------------------------------------------------------------------
@@ -592,7 +592,7 @@
 # Read arch specific Makefile to set KBUILD_DEFCONFIG as needed.
 # KBUILD_DEFCONFIG may point out an alternative default configuration
 # used for 'make defconfig'
-include arch/$(SRCARCH)/Makefile
+include $(srctree)/arch/$(SRCARCH)/Makefile
 export KBUILD_DEFCONFIG KBUILD_KCONFIG CC_VERSION_TEXT
 
 config: outputmakefile scripts_basic FORCE
@@ -679,7 +679,7 @@
 export RETPOLINE_CFLAGS
 export RETPOLINE_VDSO_CFLAGS
 
-include arch/$(SRCARCH)/Makefile
+include $(srctree)/arch/$(SRCARCH)/Makefile
 
 ifdef need-config
 ifdef may-sync-config