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.
Having configuration.nix generation hidden underneath nixos-option
never made sense, also given that there was another command to
generate part of the configuration (nixos-hardware-scan). Now
nixos-generate-config produces both configuration.nix and
hardware-configuration.nix. The latter is overwritten while the
former is not.
