bug#51547: Erase / on boot

[Tom Fitzhenry]

This issue tracks the creation of a Guix System implementation of 
https://grahamc.com/blog/erase-your-darlings :

    "I erase my systems at every boot.
    [...]
    NixOS can boot with only two directories: /boot, and /nix."

I have a working prototype of 
https://elis.nu/blog/2020/05/nixos-tmpfs-as-root/ . I will submit some 
small fixes in the short term, and later some larger patches.

Ideally this will result in a cookbook entry, and a CI test.