mkimage: Add -k option to specify key directory

Keys required for signing images will be in a specific directory. Add a
-k option to specify that directory.

Also update the mkimage man page with this information and a clearer list
of available commands.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de> (v1)