mkimage: Show item lists for all categories

Update the error-handling code for -A, -C and -O to show a list of valid
options when an invalid one is provided.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Vinoth Eswaran <evinoth1206@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>