nitrogen6x: config: allow boot to USB stick
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>
Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>