updatable-image.bbclass: add on_app_image()
updatable-image.bbclass: add on_app_image()

To allow users of `updatable-image` class to extend only applications
images - but regardless of other influencing modifiers (e.g. -dev),
introduce a check function which proves whether it's an `-app-*image`
or not.

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

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