imx_common: check for Serial Downloader in spl_boot_device

imx_common: check for Serial Downloader in spl_boot_device

Check for bmode before reading the boot device
to check if a serial downloader is started,
and returns UART if the serial downloader is set,
letting SPL to wait for an image if
CONFIG_SPL_YMODEM_SUPPORT is set.

This allows to load again a SPL based board
with imx_usb_loader together with a tool
such as kermit.

Signed-off-by: Stefano Babic <[email protected]>
CC: Tim Harvey <[email protected]>
CC: Fabio Estevam <[email protected]>
CC: Eric Nelson <[email protected]>
Reviewed-by: Eric Nelson <[email protected]>
Tested-by: Eric Nelson <[email protected]>