kbuild: rename OBJTREE to objtree

Prior to Kbuild, $(OBJTREE) was used for pointing to the
top of build directory with absolute path.

In Kbuild style, $(objtree) is used instead.
This commit renames OBJTREE to objtree and delete the
defition of OBJTREE.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/mkconfig b/mkconfig
index 17ff298..cd911a9 100755
--- a/mkconfig
+++ b/mkconfig
@@ -104,9 +104,9 @@
 # Create link to architecture specific headers
 #
 if [ -n "$KBUILD_SRC" ] ; then
-	mkdir -p ${OBJTREE}/include
+	mkdir -p ${objtree}/include
 	LNPREFIX=${srctree}/arch/${arch}/include/asm/
-	cd ${OBJTREE}/include
+	cd ${objtree}/include
 	mkdir -p asm
 else
 	cd arch/${arch}/include