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.host

@@ -71,7 +71,7 @@ __hostc_flags	= $(_hostc_flags)
 __hostcxx_flags	= $(_hostcxx_flags)
 
 ifeq ($(KBUILD_EXTMOD),)
-ifneq ($(KBUILD_SRC),)
+ifneq ($(srctree),.)
 __hostc_flags	= -I$(obj) $(call flags,_hostc_flags)
 __hostcxx_flags	= -I$(obj) $(call flags,_hostcxx_flags)
 endif