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/config.mk b/config.mk
index 9bf4eda..05864aa 100644
--- a/config.mk
+++ b/config.mk
@@ -25,7 +25,7 @@
 # Check if arch/$ARCH/cpu/$CPU exists, otherwise assume arch/$ARCH/cpu contains
 # CPU-specific code.
 CPUDIR=arch/$(ARCH)/cpu/$(CPU)
-ifneq ($(SRCTREE)/$(CPUDIR),$(wildcard $(SRCTREE)/$(CPUDIR)))
+ifneq ($(srctree)/$(CPUDIR),$(wildcard $(srctree)/$(CPUDIR)))
 CPUDIR=arch/$(ARCH)/cpu
 endif