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.build b/scripts/Makefile.build
index 1b6094a..750d6d5 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -35,27 +35,27 @@
 # Read auto.conf if it exists, otherwise ignore
 -include include/config/auto.conf
 
-include scripts/Kbuild.include
+include $(srctree)/scripts/Kbuild.include
 
 # The filename Kbuild has precedence over Makefile
 kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))
 kbuild-file := $(if $(wildcard $(kbuild-dir)/Kbuild),$(kbuild-dir)/Kbuild,$(kbuild-dir)/Makefile)
 include $(kbuild-file)
 
-include scripts/Makefile.lib
+include $(srctree)/scripts/Makefile.lib
 
 # Do not include hostprogs rules unless needed.
 # $(sort ...) is used here to remove duplicated words and excessive spaces.
 hostprogs := $(sort $(hostprogs))
 ifneq ($(hostprogs),)
-include scripts/Makefile.host
+include $(srctree)/scripts/Makefile.host
 endif
 
 # Do not include userprogs rules unless needed.
 # $(sort ...) is used here to remove duplicated words and excessive spaces.
 userprogs := $(sort $(userprogs))
 ifneq ($(userprogs),)
-include scripts/Makefile.userprogs
+include $(srctree)/scripts/Makefile.userprogs
 endif
 
 ifndef obj