Issue description

I'd like to nixos-rebuild switch --rollback system to an old configuration, but current interface allows rollback only to previous one.

In NixOps, for-example, you can specify the generation number you'd like to rollback to. The generations and their numbers are shown with nixops list-generations

Doing things manually show an error:

# ln -sf /nix/store/5y0ypnyz6mrh95a71qk82kx5cwxfy1dl-nixos-system-nixos-16.09.773.b8ede35 /nix/var/nix/profiles/system
ln: failed to create symbolic link '/nix/var/nix/profiles/system/5y0ypnyz6mrh95a71qk82kx5cwxfy1dl-nixos-system-nixos-16.09.773.b8ede35': Read-only file system

Technical details

• System: nixos-16.09