OpenSystemsDevelopment / usbboot

This also resolve a startup issue CM4-S

Ideally, the bootloader EEPROM should be updated before using
bootcode4 to run Linux so that full SDRAM init can be loaded.
This version fails over to a generic slow configuration if BRCM
DDR FW is not found.

Create a new rpiboot mode that uses a Linux initramfs to
implement mass-storage-gadget mode supporting EMMC and NVMe
block devices.

To use:
cd usbboot/mass-storage/gadget
../rpiboot -d .