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/Makefile b/Makefile
index f18f3eb..a8ca5e9 100644
--- a/Makefile
+++ b/Makefile
@@ -168,10 +168,9 @@
 OBJTREE		:= $(objtree)
 SPLTREE		:= $(OBJTREE)/spl
 TPLTREE		:= $(OBJTREE)/tpl
-SRCTREE		:= $(srctree)
-export	SRCTREE OBJTREE SPLTREE TPLTREE
+export	OBJTREE SPLTREE TPLTREE
 
-MKCONFIG	:= $(SRCTREE)/mkconfig
+MKCONFIG	:= $(srctree)/mkconfig
 export MKCONFIG
 
 # Make sure CDPATH settings don't interfere