Merge commit '3ab2949' from staging into master

Conflicts:
	pkgs/development/compilers/llvm/6/llvm.nix
	pkgs/servers/home-assistant/component-packages.nix

nixos/modules/misc/nixpkgs.nix

@@ -61,7 +61,7 @@ in
             inherit (config.nixpkgs) config overlays system;
           }
         '';
-      default = import ../../.. { inherit (cfg) config overlays system; };
+      default = import ../../.. { inherit (cfg) config overlays system crossSystem; };
       type = pkgsType;
       example = literalExample ''import <nixpkgs> {}'';
       description = ''
@@ -130,6 +130,18 @@ in
       '';
     };
 
+    crossSystem = mkOption {
+      type = types.nullOr types.attrs;
+      default = null;
+      description = ''
+        The description of the system we're cross-compiling to, or null
+        if this isn't a cross-compile. See the description of the
+        crossSystem argument in the nixpkgs manual.
+
+        Ignored when <code>nixpkgs.pkgs</code> is set.
+      '';
+    };
+
     system = mkOption {
       type = types.str;
       example = "i686-linux";