kbuild: rename TOPDIR to stctree

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 instead.
This commit renames TOPDIR to srctree and delete the
defition of TOPDIR.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/doc/README.SPL b/doc/README.SPL
index b1bc3ca..57a39a4 100644
--- a/doc/README.SPL
+++ b/doc/README.SPL
@@ -13,7 +13,7 @@
 How it works
 ------------
 
-There is a new directory TOPDIR/spl which contains only a Makefile.
+There is a new directory $(srctree)/spl which contains only a Makefile.
 The object files are built separately for SPL and placed in this directory.
 The final binaries which are generated are u-boot-spl, u-boot-spl.bin and
 u-boot-spl.map.