This relies on users using `nixpkgs.lib.nixosSystem` to define their
system; otherwise, the `vm` and `vmWithBootLoader` attributes will not
exist.

(cherry picked from commit e26b348689)

This preserves the nixosSystem function if another flake uses lib.extend
itself.

We built at least armv7l on hydra, therefor nixpkgs should also expose it.

Nixpkgs takes up a lot of disk space so we shouldn't do this by
default.

This allows you to specify the system-wide flake registry. One use is
to pin 'nixpkgs' to the Nixpkgs version used to build the system:

  nix.registry.nixpkgs.flake = nixpkgs;

where 'nixpkgs' is a flake input. This ensures that commands like

  $ nix run nixpkgs#hello

pull in a minimum of additional store paths.

You can also use this to redirect flakes, e.g.

  nix.registry.nixpkgs.to = {
    type = "github";
    owner = "my-org";
    repo = "my-nixpkgs";
  };

Dirty trees no longer set 'rev', so we need to handle this.

E.g.

  $ nixos-version
  19.03.20190913.af5eb77 (Koi)

Flakes now are evaluated in pure mode, so we can't rely on
currentSystem anymore.