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

Non-empty $(KBUILD_SRC) means out-of-tree build.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/mkconfig b/mkconfig
index 9827e4d..b755d2a 100755
--- a/mkconfig
+++ b/mkconfig
@@ -103,7 +103,7 @@
 #
 # 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 @@
 	ln -s ${LNPREFIX}proc-armv asm/proc
 fi
 
-if [ "$SRCTREE" = "$OBJTREE" ] ; then
+if [ -z "$KBUILD_SRC" ] ; then
 	cd ${SRCTREE}/include
 fi