ARMv7: usbboot can now handle arbitrary entry points.

This means that the entry point no longer needs to be at the start of the
loaded segment - usbboot will read the entry point from the ELF, and pass that
to the second-stage loader. This removes a restriction on ARMv7 boot driver
linking, and saves a little space.

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

ARMv7: Made some improvements to usbboot.

The bulk transfer now incorporates acknowledgements, which seems to help keep
the two ends in synch, and I was able to push the chunk size to 64k, for a
marginal speed improvement. More significantly, there are now fewer hardcoded
sleeps, so the overall boot process is faster.

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

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>