nitrogen6x: Allow boot to USB mass storage

This patch enables boot to USB storage devices by expanding on the list
of boot devices.

Because the USB startup currently takes a long time, it places USB at
the end of the list of supported devices.

You can over-ride the boot order using the bootdevs environment variable.
For instance, this will make USB the first (highest priority) device:

	U-Boot > setenv bootdevs usb mmc sata
	U-Boot > saveenv

Signed-off-by: Diego Rondini <diego.rondini@kynetics.it>