kbuild: Fix checking of scm-identifier variable

I'm looking Makefile in the -mm branch (dated 2010-04-28-16-53) and
seeing what looks like a bug in the checking of scm-identifier.  The
"ifneq ($scm-identifier)" seems to always execute "ifeq
($(LOCALVERSION,)) ...".  This patch fixes the checking of
scm-identifier.

Signed-off-by: Greg Thelen <gthelen@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>