uboot-phytec

pico-imx6ul: Add bmode support

The 'bmode' command is helpful for switching the boot media.

In the case of pico-imx6ul there are two possible boot media:
eMMC or USB.

To boot from eMMC:

=> bmode emmc

To boot from USB (via Serial Download Protocol):

=> bmode usb

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Fabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>