Add a jobset for a "nixos-small" channel

This channel only builds a small subset of Nixpkgs, mostly suitable
for servers. Since the channel update doesn't require thousands of
packages to be built first, it should provide much faster turnaround
in case of security updates.

(cherry picked from commit 2c7acc6731)

nixos/release-small.nix

@@ -0,0 +1,85 @@
+# This jobset is used to generate a NixOS channel that contains a
+# small subset of Nixpkgs, mostly useful for servers that need fast
+# security updates.
+
+{ nixpkgs ? { outPath = ./..; revCount = 56789; shortRev = "gfedcba"; }
+, stableBranch ? false
+, supportedSystems ? [ "x86_64-linux" ] # no i686-linux
+}:
+
+let
+
+  nixpkgsSrc = nixpkgs; # urgh
+
+  pkgs = import ./.. {};
+
+  lib = pkgs.lib;
+
+  nixos' = import ./release.nix {
+    inherit stableBranch supportedSystems;
+    nixpkgs = nixpkgsSrc;
+  };
+
+  nixpkgs' = builtins.removeAttrs (import ../pkgs/top-level/release.nix {
+    inherit supportedSystems;
+    nixpkgs = nixpkgsSrc;
+  }) [ "unstable" ];
+
+in rec {
+
+  nixos = {
+    inherit (nixos') channel manual iso_minimal dummy;
+    tests = {
+      inherit (nixos'.tests)
+        containers
+        firewall
+        ipv6
+        login
+        misc
+        nat
+        nfs3
+        openssh
+        proxy
+        simple;
+      installer = {
+        inherit (nixos'.tests.installer)
+          grub1
+          lvm
+          separateBoot
+          simple;
+      };
+    };
+  };
+
+  nixpkgs = {
+    inherit (nixpkgs')
+      apacheHttpd_2_2
+      apacheHttpd_2_4
+      emacs
+      gd
+      mysql51
+      mysql55
+      nginx
+      openssh
+      php
+      postgresql92
+      postgresql93
+      python
+      rsyslog
+      stdenv
+      tarball
+      vim;
+  };
+
+  tested = pkgs.releaseTools.aggregate {
+    name = "nixos-${nixos.channel.version}";
+    meta = {
+      description = "Release-critical builds for the NixOS channel";
+      maintainers = [ lib.maintainers.eelco ];
+    };
+    constituents =
+      let all = x: map (system: x.${system}) supportedSystems; in
+      [ nixpkgs.tarball ] ++ lib.collect lib.isDerivation nixos;
+  };
+
+}