uboot-imx

spl: Rework image header parse to allow abort on raw image and os boot

This reworks spl_set_header_raw_uboot to allow having both os boot
(which comes with a valid header) and aborting when no valid header is
found (thus excluding raw u-boot.bin images).

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@konsulko.com>