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/spl/Makefile b/spl/Makefile
index 640db8b..b89a579 100644
--- a/spl/Makefile
+++ b/spl/Makefile
@@ -68,7 +68,7 @@
 include/spl-autoconf.mk: include/config.h
 	$(call cmd,autoconf)
 
-HAVE_VENDOR_COMMON_LIB = $(if $(wildcard $(SRCTREE)/board/$(VENDOR)/common/Makefile),y,n)
+HAVE_VENDOR_COMMON_LIB = $(if $(wildcard $(srctree)/board/$(VENDOR)/common/Makefile),y,n)
 
 ifdef	CONFIG_SPL_START_S_PATH
 START_PATH := $(CONFIG_SPL_START_S_PATH:"%"=%)
@@ -145,7 +145,7 @@
 # Linker Script
 ifdef CONFIG_SPL_LDSCRIPT
 # need to strip off double quotes
-LDSCRIPT := $(addprefix $(SRCTREE)/,$(CONFIG_SPL_LDSCRIPT:"%"=%))
+LDSCRIPT := $(addprefix $(srctree)/,$(CONFIG_SPL_LDSCRIPT:"%"=%))
 endif
 
 ifeq ($(wildcard $(LDSCRIPT)),)