[new uImage] POWERPC: Add image_get_fdt() routine

FDT blob may be passed either: (1) raw (2) or embedded in the legacy uImage
(3) or embedded in the new uImage. For the (2) case embedding image must be
verified before we get FDT from it. This patch factors out legacy image
specific verification routine to the separate helper routine.

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