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>