sunxi: Properly announce BOOT_DEVICE_BOARD as "FEL"

This addresses a cosmetic issue when booting a sunxi device
over USB (FEL mode), where the SPL currently would just print
"Trying to boot from ". The patch fixes that to properly read
"Trying to boot from FEL".

Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>