[new uImage] Optimize gen_get_image() flow control

When CONFIG_HAS_DATAFLASH is not defined gen_get_image() routine has nothing
to do, update its control flow to better reflect that simple case.

Signed-off-by: Marian Balakowicz <m8@semihalf.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>