OpenSystemsDevelopment / usbboot

* Remove the reference to LEDs flashing with mass-storage-gadget
  because this code was removed.
* Mention LUKS / rpi-otp-private-key in the secure-boot section.
* Add a link to the PIP
* Drop reference to make-boot-image - just use buildroot.

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 .