imgtool: Add --public-key-format option to imgtool

The --public-key-format option can be used to distinguish where
the public key is stored for image authentication. It can be embedded
in MCUboot or by selecting 'full' it can be in the image manifest.

The source of this change:
https://review.trustedfirmware.org/c/trusted-firmware-m/+/1579



Change-Id: If658dff8147cfce2f27bfc0209ecf0d6d9cb2a73
Signed-off-by: David Vincze <david.vincze@linaro.org>