imx7: spl: Check for Serial Downloader in spl_boot_device

imx7: spl: Check for Serial Downloader in spl_boot_device

Similarly to imx6, before reading the boot device, first check
bmode to see if the serial downloader has been selected
explicitly, then check whether the serial downloader has been
activated due to unbootable primary boot devices (e.g. empty eMMC).

If the serial downloader is activated, return BOOT_DEVICE_BOARD.
This allows SPL with SDP support to wait for the U-Boot image
to be loaded via the serial download protocol using imx_usb_loader.

Signed-off-by: Eran Matityahu <eran.m@variscite.com>