kbuild: disable depmod in cross-compile kernel build

When building embedded systems in a cross-compile environment and
populating a target's file system image, we don't want to run the
depmod on the host as we may be building for a completely different
architecture. Since there's no such thing as a cross-depmod, we
just disable running depmod in the cross-compile case and we just
run depmod on the target at bootup.

Inspired by patches from Christian, Armin and Deepak.

This solves: http://bugzilla.kernel.org/show_bug.cgi?id=3881

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Christian Bjølevik <nafallo@magicalforest.se>
Cc: Deepak Saxena <dsaxena@mvista.com> and
Cc: Armin Kuster <akuster@mvista.com>,