kbuild: rename SRCTREE to srctree

Prior to Kbuild, $(TOPDIR) or $(SRCTREE) was used for
pointing to the top of source directory.
(No difference between the two.)

In Kbuild style, $(srctree) is used for instead.
This commit renames SRCTREE to srctree and deletes the
defition of SRCTREE.

Note that SRCTREE in scripts/kernel-doc, scripts/docproc.c,
doc/DocBook/Makefile should be keep.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/mkconfig b/mkconfig
index b755d2a..17ff298 100755
--- a/mkconfig
+++ b/mkconfig
@@ -105,7 +105,7 @@
 #
 if [ -n "$KBUILD_SRC" ] ; then
 	mkdir -p ${OBJTREE}/include
-	LNPREFIX=${SRCTREE}/arch/${arch}/include/asm/
+	LNPREFIX=${srctree}/arch/${arch}/include/asm/
 	cd ${OBJTREE}/include
 	mkdir -p asm
 else
@@ -126,7 +126,7 @@
 fi
 
 if [ -z "$KBUILD_SRC" ] ; then
-	cd ${SRCTREE}/include
+	cd ${srctree}/include
 fi
 
 #