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.asm-generic b/scripts/Makefile.asm-generic
index 82ad63d..1d501c57 100644
--- a/scripts/Makefile.asm-generic
+++ b/scripts/Makefile.asm-generic
@@ -14,10 +14,10 @@
 
 # $(generic)/Kbuild lists mandatory-y. Exclude um since it is a special case.
 ifneq ($(SRCARCH),um)
-include $(generic)/Kbuild
+include $(srctree)/$(generic)/Kbuild
 endif
 
-include scripts/Kbuild.include
+include $(srctree)/scripts/Kbuild.include
 
 redundant := $(filter $(mandatory-y) $(generated-y), $(generic-y))
 redundant += $(foreach f, $(generic-y), $(if $(wildcard $(srctree)/$(src)/$(f)),$(f)))