mkimage: fix argument parsing on BSD systems

The getopt(3) optstring '-' is a GNU extension which is not available on BSD
systems like OS X.

Remove this dependency by implementing argument parsing in another way. This
will also change the lately introduced '-b' switch behaviour.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>