mkimage: fix display of image types list

Since commit 5b9d44df (mkimage: Display a better list of available image
types) mkimage usage text suggest to "use -T to see a list of available image
types". Unfortunately, commit 02221f29deb8 (mkimage: Convert to use getopt())
broke that feature, because getopt() fails when -T has no option argument.

Add a pseudo image type name 'list' that lists all image types. Update the
usage text accordingly.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>