initrd: Optional systemd-based initrd

nixos/lib/systemd-lib.nix

@@ -291,16 +291,10 @@ in rec {
     };
   };
 
-  serviceConfig = { name, config, ... }: {
+  mkServiceConfig = path: { name, config, ... }: {
     config = mkMerge
       [ { # Default path for systemd services.  Should be quite minimal.
-          path = mkAfter
-            [ pkgs.coreutils
-              pkgs.findutils
-              pkgs.gnugrep
-              pkgs.gnused
-              systemd
-            ];
+          path = mkAfter path;
           environment.PATH = "${makeBinPath config.path}:${makeSearchPathOutput "bin" "sbin" config.path}";
         }
         (mkIf (config.preStart != "")
@@ -330,6 +324,16 @@ in rec {
       ];
   };
 
+  serviceConfig = mkServiceConfig [
+    pkgs.coreutils
+    pkgs.findutils
+    pkgs.gnugrep
+    pkgs.gnused
+    systemd
+  ];
+
+  initrdServiceConfig = mkServiceConfig [];
+
   mountConfig = { config, ... }: {
     config = {
       mountConfig =
@@ -387,6 +391,15 @@ in rec {
         '';
     };
 
+  initrdServiceToUnit = name: def:
+    { inherit (def) aliases wantedBy requiredBy enable;
+      text = commonUnitText def +
+        ''
+          [Service]
+          ${attrsToSection def.serviceConfig}
+        '';
+    };
+
   socketToUnit = name: def:
     { inherit (def) aliases wantedBy requiredBy enable;
       text = commonUnitText def +