mn10300: replace deprecated "TOPDIR" with newer "srctree"

This would appear to be the last reference to TOPDIR in the entire tree, after
which i'm guessing that variable can be dropped.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>