USB MSD device mode drivers for signed-boot

If secure-boot has been enabled then this image must be signed with the customer's RSA private key. Otherwise, the SPI EEPROM bootloader will refused to load this image.

To do this run:

KEY_FILE=$HOME/private.pem
../tools/rpi-eeprom-digest -i boot.img -o boot.sig -k "${KEY_FILE}"

To run load the USB MSD device drivers via RPIBOOT run

../rpiboot -d .