At the current stage, the USB HID and storage drivers are prinicpally
working but not stable. If interrupts are not processed fast enough,
devices will get sporadically disconnected.
The USB host-controller driver is not part of the normal Linux kernel.
For this reason, we need to download it separately. There exists a
'prepare_rpi' rule in the 'dde_linux/Makefile' to automate this process.
