Phew.

This was mostly blocked by Micropython, but it has since then also
upgraded to IDF 5: https://github.com/micropython/micropython/commit/e4650125b88a35f074097f16d84a8f49bd22ac06

We take that change and selectively apply it where necessary. However,
it's not a 1:1 patch apply, as we already were on a somewhat forked
Micropython, and we also want to support IDF 5.1, not 5.0.

We also bump up the IDF distribution brought in by Nix, and fix some
warnings that have since turned into errors due to toolchain changes.

This adds initial support for building on Nix(OS). It can be tested by
following the updated README.md. The result should be the ability to
build the firmware without having to download any other dependencies.