kbuild: use $(srctree) instead of KBUILD_SRC to check out-of-tree build

KBUILD_SRC was conventionally used for some different purposes:
 [1] To remember the source tree path
 [2] As a flag to check if sub-make is already done
 [3] As a flag to check if Kbuild runs out of tree

For [1], we do not need to remember it because the top Makefile
can compute it by $(realpath $(dir $(lastword $(MAKEFILE_LIST))))

[2] has been replaced with self-commenting 'sub_make_done'.

For [3], we can distinguish in-tree/out-of-tree by comparing
$(srctree) and '.'

This commit converts [3] to prepare for the KBUILD_SRC removal.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.lib

@@ -144,7 +144,7 @@ __cpp_flags     = $(_cpp_flags)
 # If building the kernel in a separate objtree expand all occurrences
 # of -Idir to -I$(srctree)/dir except for absolute paths (starting with '/').
 ifeq ($(KBUILD_EXTMOD),)
-ifneq ($(KBUILD_SRC),)
+ifneq ($(srctree),.)
 
 # -I$(obj) locates generated .h files
 # $(call addtree,-I$(obj)) locates .h files in srctree, from generated .c files