Writable filesystem support

[ikeycode]

Tracking as a low-priority issue for writable filesystem support. Basic gist being we want to be able to produce images (and potentially support driver implementations) purely from safe Rust without needing root.

Marking as low priority because frankly it'll require some hardware investments and hours from me which in turn means outside investment. Realistically we want some portability to support being OS agnostic for any given CI runner or developers environment.