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

Non-empty $(KBUILD_SRC) means out-of-tree build.
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>

Makefile

@@ -556,11 +556,9 @@ export CONFIG_SYS_TEXT_BASE
 
 # Use UBOOTINCLUDE when you must reference the include/ directory.
 # Needed to be compatible with the O= option
-UBOOTINCLUDE    :=
-ifneq ($(OBJTREE),$(SRCTREE))
-UBOOTINCLUDE	+= -I$(OBJTREE)/include
-endif
-UBOOTINCLUDE	+= -I$(srctree)/include \
+UBOOTINCLUDE    := \
+		-Iinclude \
+		$(if $(KBUILD_SRC), -I$(srctree)/include) \
 		-I$(srctree)/arch/$(ARCH)/include
 
 NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)

mkconfig

@@ -103,7 +103,7 @@ fi
 #
 # Create link to architecture specific headers
 #
-if [ "$SRCTREE" != "$OBJTREE" ] ; then
+if [ -n "$KBUILD_SRC" ] ; then
 	mkdir -p ${OBJTREE}/include
 	LNPREFIX=${SRCTREE}/arch/${arch}/include/asm/
 	cd ${OBJTREE}/include
@@ -125,7 +125,7 @@ if [ "${arch}" = "arm" ] ; then
 	ln -s ${LNPREFIX}proc-armv asm/proc
 fi
 
-if [ "$SRCTREE" = "$OBJTREE" ] ; then
+if [ -z "$KBUILD_SRC" ] ; then
 	cd ${SRCTREE}/include
 fi