mkimage: Display a better list of available image types

Offer to display the available image types in help. Also, rather than
hacking the genimg_get_type_id() function to display a list of types,
do this in the tool. Also, sort the list.

The list of image types is quite long, and hard to discover. Print it out
when we show help information.

Signed-off-by: Simon Glass <sjg@chromium.org>