image_types: Refactor compression code into a generic compression solution
The current approach of adding each different compressed image type doesn't
scale. This patch changes the code so compressed images for each form are
automatically available using the form <type>.<compression type> in
IMAGE_FSTYPES. This doesn't change any existing externally visible behaviour
and the image generation process becomes more efficient as a result too.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
