This patch adds support for booting from UAS (usb
attached scsi) devices.
For now only usb 2.0 support is there. On usb 3.0 the
UAS protocol uses streams, so changes will be required
to make usb 3.0 devices fly once we have a xhci host
controller driver.
So far the driver has been tested on qemu-emulated
virtual hardware only. In theory should just work on
bare metal too.