Raspberry Pi USB booting code

 USB MSD boot code which should work on the Raspberry Pi model A, Compute
 Module, Compute Module 3, Compute Module 4, Zero, Zero W, and Zero 2.
 .
 Also includes utilities for working with secure boot on the Raspberry Pi 4
 series (including Compute Module 4 and certain board revisions of the 4B and
 400).