the root part is ext4 instead of btrfs. nixos-generators doesn't support
btrfs. the underlying machinery does though, so we can remove the
middleman in a future patch to achieve that.
this requires cross compilation (particularly of the kernel), which is a bit too slow on this machine
to test since it uses qemu.
i can maybe switch to an older nixpkgs for the pda build which has the
kernel cached... or migrate my desktop to nixos and build the pinephone
image there :-)
