move o.systemConfiguration to initramfs module

as far as I can tell, we define it identically in every module
that uses initramfs

modules/outputs/ext4fs.nix

@@ -20,8 +20,6 @@ in
     };
     boot.initramfs.enable = true;
     system.outputs = {
-      systemConfiguration =
-        pkgs.systemconfig config.filesystem.contents;
       rootfs =
         let
           inherit (pkgs.pkgsBuildBuild) runCommand e2fsprogs;

modules/outputs/initramfs.nix

@@ -53,6 +53,8 @@ in
           file /init ${pkgs.preinit}/bin/preinit 0755 0 0
           SPECIALS
         '';
+      systemConfiguration =
+        pkgs.systemconfig config.filesystem.contents;
     };
   };
 }

modules/outputs/jffs2.nix

@@ -24,8 +24,6 @@ in
     };
     boot.initramfs.enable = true;
     system.outputs = {
-      systemConfiguration =
-        pkgs.systemconfig config.filesystem.contents;
       rootfs =
         let
           inherit (pkgs.pkgsBuildBuild) runCommand mtdutils;

modules/outputs/ubifs.nix

@@ -26,8 +26,6 @@ in
     };
     boot.initramfs.enable = true;
     system.outputs = {
-      systemConfiguration =
-        pkgs.systemconfig config.filesystem.contents;
       rootfs =
         let
           inherit (pkgs.pkgsBuildBuild) runCommand mtdutils;