fastboot: Implement OEM format only when we have MMC support

The current fastboot support assumes that CONFIG_FASTBOOT_FLASH implies
that we have an MMC in our system, which might not be the case if we have
some other storage device.

Change the configuration option protecting that call to
FASTBOOT_FLASH_MMC_DEV, that makes much more sense.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>