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.commands b/doc/README.commands
index 9eb3671..afd5577 100644
--- a/doc/README.commands
+++ b/doc/README.commands
@@ -28,7 +28,7 @@
 referenced in the source code as such.
 
 If a new board is defined do not forget to define the command section
-by writing in u-boot.lds ($(TOPDIR)/board/boardname/u-boot.lds) these
+by writing in u-boot.lds ($(srctree)/board/boardname/u-boot.lds) these
 3 lines:
 
 	.u_boot_list : {