nixos-generate-config can write incorrect unionfs

[cyounkins]

Issue description

As of NixOS 18.09 nixos-generate-config can write an incorrect unionfs mount when used on the livecd without --root /mnt

  fileSystems."/nix/store" =
    { device = "unionfs";
      fsType = "fuse.unionfs";
    };

Here is the whole thing: https://gist.github.com/cyounkins/7b56d15576935cda5102be46edf43c4f

Steps to reproduce

1. Boot the livecd
2. nixos-generate-config
3. Observe the written hardware configuration

It should be incorrect to specify device = "unionfs" without union directories. Maybe we could also warn if we think the invocation is incorrect.

Some authors of the script: @edolstra @wizeman

[stale]

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