mkimage: Add long options

The mkimage command has had many options added over the years.
Unfortunately, we are starting to run out of short options. Recent options
don't have any obvious relation to their meaning (e.g. -o/-g). Fortunately,
long options exist. Add long options for each current short option.

For the curious, the remaining short options are HIkLmMPQSuUwWXyYzZ.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>