Add NixOS support

[nothingnesses]

Support for NixOS would be nice. I tried using the generic loopback.cfg but it didn't work properly as the OS was expecting a certain directory hierarchy which wasn't present. To make this work, I guess the cfg would have to somehow let the OS know where to find the directories it expects. Hopefully someone smarter than me can figure out how to do this.

[xfnw]

i managed to get it to boot from the generic loopback with some changes:

nixos seems to depend on the bootloader knowing some stuff about which derivation to boot to via kernal parameters, you need to copy the init= kernel parameter of /isolinux/isolinux.cfg from the iso into bootoptions, which is different for every new nixos iso because of the store's hashes

linux can be set to (loop)/boot/bzImage $bootoptions, and initrd to (loop)/boot/initrd