ARMv7: Made some improvements to usbboot.

Customised a bit for Barrelfish now, and supports loading from an ELF without
specifying an explicit base address. The existing code was also chopping the
last few hundred bytes off of every image it loaded.

Signed-off-by: David Cock <david.cock@inf.ethz.ch>

ARMv7: Brought usbboot into the Barrelfish tree.

We can only boot with a customised version anyway, and this way we are forced
to maintain it, and users don't have to go trawling github for the right
version. It's BSD licensed.

Signed-off-by: David Cock <david.cock@inf.ethz.ch>