Implement switching root.

[samueldr]

That is, going to stage-2.

The first step is to manage it like lxc containers, where an omnipresent init points to the last generation marked to boot. This will make it possible to do stage-2 without having to manage existing generations.

[samueldr]

This is partially merged to master, due to merging wip/cross.

The current implementation is a bodge, at best.

[samueldr]

Just a note, the code should be working, but I haven't been able yet to test on hardware due to miscellaneous issues.

• Building a stage-2 image for armv7 is hard (though I could use an existing one.
• Flashing the data somewhere on sd-card-less devices is an issue not resolved yet.
• One device is now (hopefully temporarily) out of service. (Dead battery)
• The remaining aarch64 device has issues, and among others, seems to not detect SD cards with the aarch64 kernel.

The current status is: indirectly related issues are stopping me from progressing.

[samueldr]

Demonstrably works on qemu target, depthcharge target.

Not obvious, but also work on android targets, the lambdaflake get colourized, meaning that it got to just before starting systemd's init, meaning that it did switch_root into the NixOS stage-2. For android, we're still left with finding useful graphical software for stage-2, but that's out of scope of this particular issue :).