updatable-image.bbclass: let user extend on-image
updatable-image.bbclass: let user extend on-image

Allow - analogous to image.bbclass - add other classes to inherit from to
add image features, packages, what ever - with additional knowledge of
updatable-image.

Signed-off-by: Jens Rehsack <69d49c504a9219a55b8be0f905e8fe3c32ffb57f@netbsd.org>

rehsack/meta-jens,rehsack/meta-jens,rehsack/meta-jens