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>

1 files changed, 3 insertions, 3 deletions

diff --git a/Makefile b/Makefile
index 4929c1f41cfa..72fad444f3e8 100644
--- a/Makefile
+++ b/Makefile
@@ -448,7 +448,7 @@ USERINCLUDE    := \
 LINUXINCLUDE    := \
                 -I$(srctree)/arch/$(SRCARCH)/include \
                 -I$(objtree)/arch/$(SRCARCH)/include/generated \
-                $(if $(KBUILD_SRC), -I$(srctree)/include) \
+                $(if $(filter .,$(srctree)),,-I$(srctree)/include) \
                 -I$(objtree)/include \
                 $(USERINCLUDE)
 
@@ -509,7 +509,7 @@ PHONY += outputmakefile
 # At the same time when output Makefile generated, generate .gitignore to
 # ignore whole output directory
 outputmakefile:
-ifneq ($(KBUILD_SRC),)
+ifneq ($(srctree),.)
         $(Q)ln -fsn $(srctree) source
         $(Q)$(CONFIG_SHELL) $(srctree)/scripts/mkmakefile $(srctree)
         $(Q)test -e .gitignore || \
@@ -1089,7 +1089,7 @@ PHONY += prepare archprepare prepare1 prepare3
 # and if so do:
 # 1) Check that make has not been executed in the kernel src $(srctree)
 prepare3: include/config/kernel.release
-ifneq ($(KBUILD_SRC),)
+ifneq ($(srctree),.)
         @$(kecho) '  Using $(srctree) as source for kernel'
         $(Q)if [ -f $(srctree)/.config -o -d $(srctree)/include/config ]; then \
                 echo >&2 "  $(srctree) is not clean, please run 'make mrproper'"; \