make-mod-scripts: Ensure kernel build output is deterministic

The definitions in linux-kernel-base are needed to ensure the generated headers
are consistent. This was a small step that was missing from the previous
changes to linux-kernel-base as both kernel-devsrc and make-mod-scripts
need this information defined consistently.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 0d79d488)
Signed-off-by: Steve Sakoman <steve@sakoman.com>