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>

configs/pico-imx6ul_defconfig

@@ -9,7 +9,6 @@ CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
-# CONFIG_CMD_BMODE is not set
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg"