Script to generate an install image and flash it over USB.

The install steps are roughly:

1. Create a single install image of u-boot, kernel and initramfs

2. Load the install image over USB

3. Start u-boot and flash u-boot, kernel and initramfs

4. Start kernel and configure USB composite gadget in serial,
   Ethernet and mass storage mode with eMMC as the mass storage
   back end. The serial console shows up as /dev/ttyACM0 on the
   host PC.

5. Once the red and green LEDs start cycling, the eMMC can be
   formatted an initialized over USB mass storage from the host PC.

Note that the NAND partitions must match with u-boot and kernel
.dts file.