gummiboot-builder: When NIXOS_INSTALL_GRUB is set and efibootmgr is used, prepend the gummiboot entry to the boot entries list
gummiboot-builder: Only read /etc/machine-id once per invocation
etc.nix: Fix style
Turn environment.etc into an attribute set

This provides a more convenient syntax and allows easier overriding.

For example,

  environment.etc = singleton
    { target = "vconsole.conf";
      source = vconsoleConf;
    };

can now be written as

  environment.etc."vconsole.conf".source = vconsoleConf;
environment.etc: Add convenience option 'text'

This allows writing

  environment.etc.hosts.text = "127.0.0.1 localhost";

instead of

  environment.etc.hosts.source = pkgs.writeText "hosts" "127.0.0.1 localhost";
Selectively allow /etc files to be disabled

For instance, if you don't want NixOS to emit /etc/hosts, you
can say:

  environment.etc.hosts.enable = false;
nixos-install: Respect host $NIX_CONF_DIR and don't assume nix.conf exists
nixos-install: Respect /home/shlevy/src:/nix/var/nix/profiles/per-user/root/channels/nixos:nixpkgs=/etc/nixos/nixpkgs:nixos=/etc/nixos/nixos:nixos-config=/etc/nixos/configuration.nix:services=/etc/nixos/services to find the nixos and nixpkgs sources, only copy the nixos channel if it exists
nixos-install: Respect the build-users-group of the host