A USB keyboard may return key codes in any of the 6 key code array
positions (issue #48). So we have to compare the latest report to
the previous one to detect new key presses and handle rollover. As
the same change is needed for all HCI types, factor out the common
code into the base driver.
If the usbdebug option is present, pause at the end of the USB scan until
a key is pressed. Otherwise, if the keyboard=usb option is present and no
USB keyboards were discovered, pause for 10 seconds. Otherwise don't pause.
