[new uImage] rework error handling so common functions don't reset

Changed image_get_ramdisk() to just return NULL on error and have
get_ramdisk() propogate that error to the caller.  It's left to the
caller to call do_reset() if it wants to.

Also moved calling do_reset() in get_fdt() and fdt_relocate() on ppc
to a common location.  In the future we will change get_fdt() and
fdt_relocate() to return success/failure and not call do_reset() at all.

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