Crate up Baremetal USB handler

The Baremetal USB handler (featured in the boot-updater, baremetal, erasure, and bootloader) code is repeated roughly four times throughout the Xous repo, in almost identical line-for-line code.

Figure out a way to condense this into a single crate to improve the maintainability of this code.

One consideration is that it's desired to not have USB serial in some configurations, which complicates the condensation of the code base.