As has been mentioned earlier, the xhci driver causes the boot
process to hang on some machines, including mine. However, it turns
out that a single keystroke on the PS/2 keyboard (the USB one had
not been probed yet) allowed the boot to continue, and I successfully
accessed a USB2 flash drive connected to one of my USB3 ports. I'm
guessing then that there is an interrupt being lost somewhere, or
perhaps a missing timeout. The hang was occurring shortly after
attaching uhub0, as indicated in the dmesg.boot below, and you can
see the umass0 device attaching to it afterwards.