This adds an integer to the descriptor of devices without uniqely
identifying information. It will reuse values that are no longer
in use, so if you remove a single device and attach a different
identical device it will appear to be the same device.
TODO: Derive uniqueness from USB port when possible. This version
will generate different descriptors for each half of a USB keyboard
that shows up twice.
Change-Id: Id05d5265e21b10cbb4ae3e30f91a89a6b0e55d63