[new uImage] Move image verify flag to bootm_headers structure

Do not pass image verification flag directly to related routines.
Simplify argument passing and move it to the bootm_header structure which
contains curently processed image specific data and is already being passed
on the argument list.

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