mkimage: Add a 'keyfile' argument for image signing

It's not always desirable to use 'keydir' and some ad-hoc heuristics
to get the filename of the signing key. More often, just passing the
filename is the simpler, easier, and logical thing to do.

Since mkimage doesn't use long options, we're slowly running out of
letters. I've chosen '-G' because it was available.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>