Setup mass storage for accessing drives

One more step towards being a good dashcam.

Plug in to your computer, open file explorer, and drag any footage you want off.
* Can we make this work on Windows, macOS, and Linux without needing to install drivers?
* This unfortunately requires 2x cables like flashing, just inserted in the other order.
* Should we expose the whole `userdata` partition or just `realdata`?

There are some natural follow-ups to this, such as making the on-disk format match the good dashcams.

Clarify the mass-storage design around the two-cable connection sequence and whether to expose the userdata or realdata partition. Define done as allowing footage to be browsed and copied on Windows, macOS, and Linux without additional drivers.

Written by the indexing model from the issue text.