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/board/avnet/fx12mm/Makefile b/board/avnet/fx12mm/Makefile
index 37fb02f..618b42f 100644
--- a/board/avnet/fx12mm/Makefile
+++ b/board/avnet/fx12mm/Makefile
@@ -8,4 +8,4 @@
 
 obj-y	+= fx12mm.o
 
-include $(SRCTREE)/board/xilinx/ppc405-generic/Makefile
+include $(srctree)/board/xilinx/ppc405-generic/Makefile