Add a boot loader tunable hw.usb.hack_defer_exploration, which if set

Add a boot loader tunable hw.usb.hack_defer_exploration, which if set
to 0 reverts to the old USB behaviour, i.e. USB keyboards should be again
usable early at boot. By default, this is set to 1 which will avoid hanging
the system on qemu and my HP compaq laptop and maybe others.

Note that this is a hack around a shortcoming in the current USB stack and
will go away once the shortcoming has been fixed.