If this option is given, nixos-generate-config will write the
configuration to /etc/nixos under the given root, and only emit
fileSystems attributes for filesystems under the given root directory.
The typically use case is at installation time, where you can say:
$ nixos-generate-config --root /mnt
which will create /mnt/etc/nixos/{configuration.nix,hardware-configuration.nix}.
Also add a --force flag to force overwriting configuration.nix.
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