This allows booting from internal eMMC or SD card just changing the
U-Boot mmc_dev variable.
In particular, setting mmc_dev to 1 will result in booting from the SD card.
Setting the variable to 0 will result in internal eMMC boot (the default).
Should the variable be unset or an error condition occur while reading
from SD card, internal MMC booting will be tried.
Signed-off-by: Enrico Mioso <mrkiko.rs@gmail.com>
