arm: spear: Return to BootROM if failing to boot from the main device

Overload the weak function board_boot_order() so that besides choosing
the main boot device, we can fallback on USB boot by returning in the
BootROM, eg. if the NOR flash is empty while it was the primary boot
medium.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Stefan Roese <sr@denx.de>