Hacky support for custom packages

This is so barely baked. It doesn't actually make this easy, it just
makes it possible. You still need to override the configure phase to
symlink in the appropriate `.ipk` file for this to work.

See astro#38 for the
proper feature request.

builder.nix

@@ -23,6 +23,8 @@
   ).packages.${packagesArch}
 # Extra OpenWRT packages (can be prefixed with "-")
 , packages ? []
+# TODO: <<< explain >>>
+, hackExtraPackages ? []
 # Include extra files
 , files ? null
 # Which services in /etc/init.d/ should be disabled
@@ -91,6 +93,9 @@ pkgs.stdenv.mkDerivation {
           ln -s $file $(basename $file)
         done
       fi
+    '' + lib.optionalString (files != null) ''
+        # copy files to avoid making etc read-only
+        cp -r --no-preserve=all ${files} files
     '';
 
   nativeBuildInputs = with pkgs; [
@@ -103,7 +108,7 @@ pkgs.stdenv.mkDerivation {
     makeArgs = lib.concatStringsSep " " [
       "-j$NIX_BUILD_CORES"
       "DISABLED_SERVICES='${lib.concatStringsSep " " disabledServices}'"
-      "PACKAGES='${lib.concatStringsSep " " packages}'"
+      "PACKAGES='${lib.concatStringsSep " " (packages ++ hackExtraPackages)}'"
       "PROFILE='${profile}'"
       "SHELL='${pkgs.runtimeShell}'"
       (lib.optionalString (files != null) "FILES='./files'")
@@ -114,9 +119,6 @@ pkgs.stdenv.mkDerivation {
   in lib.optionalString (lib.versionOlder release "19") ''
     # Hack around broken check for gcc
     touch staging_dir/host/.prereq-build
-  '' + lib.optionalString (files != null) ''
-      # copy files to avoid making etc read-only
-      cp -r --no-preserve=all ${files} files
   '' + ''
     make image ${makeArgs}
   '';