uboot-phytec

imx: boot_mode: add USB_BOOT entry

Add USB_BOOT entry.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>