Add support for Pi4 (#42)
Add support for Pi4 (#42)

The BCM2711 can also boot in USB device mode. Note that the boot ROM
code now sets the iSerialNumber to 3, but it does not expect the
second stage boot server, instead it downloads a single image over
USB.

Signed-off-by: Petr Tesarik <c327e92ea10f7773037429ba850de356a617881c@tesarici.cz>
raspberrypi/usbboot,raspberrypi/usbboot,raspberrypi/usbboot