b/12068020 Add a way to add uniqueness to device descriptors

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