OpenSystemsDevelopment / usbboot

Merge https://github.com/timg236/cm4-scripts/pull/1 to remove the
LED activity flashing because this conflicts with EMMC_EN_N_1V8
on CM3.

I've reverted the change that removed the explict modprobe of
libcomposite becuase that broke the gadgetfs support for me on CM4.

Force device/peripheral mode instead of relying on the OTG pin
because this might not work on CM4S.

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 .