image/image-live: Add back IMAGE_TYPES_MASKED support
IMAGE_TYPES_MASKED support was accidentally removed. The original
idea behind it was to remove some of the hardcoding in the core
image code, so do that for image-live and ensure the dependency
and masked variables correctly reflect the needs of the class.
This means we can remove all the hardcoded special cases since
image-vm already has the needed markup.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
