systemd: allow custom unit folders to be configured with SYSTEMD_UNIT_PATH

nixos/modules/system/boot/stage-2-init.sh

@@ -167,6 +167,7 @@ exec {logOutFd}>&- {logErrFd}>&-
 
 # Start systemd.
 echo "starting systemd..."
+
 PATH=/run/current-system/systemd/lib/systemd:@fsPackagesPath@ \
-    LOCALE_ARCHIVE=/run/current-system/sw/lib/locale/locale-archive \
+    LOCALE_ARCHIVE=/run/current-system/sw/lib/locale/locale-archive @systemdUnitPathEnvVar@ \
     exec @systemdExecutable@

nixos/modules/system/boot/stage-2.nix

@@ -10,7 +10,7 @@ let
     src = ./stage-2-init.sh;
     shellDebug = "${pkgs.bashInteractive}/bin/bash";
     shell = "${pkgs.bash}/bin/bash";
-    inherit (config.boot) systemdExecutable;
+    inherit (config.boot) systemdExecutable extraSystemdUnitPaths;
     isExecutable = true;
     inherit (config.nix) readOnlyStore;
     inherit useHostResolvConf;
@@ -20,6 +20,10 @@ let
       pkgs.util-linux
     ] ++ lib.optional useHostResolvConf pkgs.openresolv);
     fsPackagesPath = lib.makeBinPath config.system.fsPackages;
+    systemdUnitPathEnvVar = lib.optionalString (config.boot.extraSystemdUnitPaths != [])
+      ("SYSTEMD_UNIT_PATH="
+      + builtins.concatStringsSep ":" config.boot.extraSystemdUnitPaths
+      + ":"); # If SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable
     postBootCommands = pkgs.writeText "local-cmds"
       ''
         ${config.boot.postBootCommands}
@@ -82,6 +86,15 @@ in
           PATH.
         '';
       };
+
+      extraSystemdUnitPaths = mkOption {
+        default = [];
+        type = types.listOf types.str;
+        description = ''
+          Additional paths that get appended to the SYSTEMD_UNIT_PATH environment variable
+          that can contain mutable unit files.
+        '';
+      };
     };
 
   };