mirrors: Add uninative mirror on kernel.org

At the last nas outage, we realised that we don't have good mirrors of the
uninative tarball if our main system can't be accessed. kernel.org mirrors
some Yocto Project data so we've ensured uninative is there. Add the appropriate
mirror url to make use of that.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>