Delay FIT format check on sector based devices

Global FIT image operations like format check cannot be performed on
a first sector data, defer them to the point when whole FIT image was
uploaded to a system RAM.

Signed-off-by: Marian Balakowicz <m8@semihalf.com>
Partial ('cmd_nand' case) Acked-by: Grant Erickson <gerickson@nuovations.com>
NAND and DOC bits Acked-by: Scott Wood <scottwood@freescale.com>