nixos/installer: add option to override the root filesystem image creator itself

allows overriding the process how the filesystem image is created which the resulting sd-card image will use.

nixos/modules/installer/sd-card/sd-image.nix

@@ -142,6 +142,15 @@ in
       '';
     };
 
+    rootFilesystemImage = mkOption {
+      type = types.package;
+      default = rootfsImage;
+      description = ''
+        The finished root partition image with all custom fileystem modifications.
+        Used to override the filesystem creator itself.
+      '';
+    };
+
     firmwareSize = mkOption {
       type = types.int;
       # As of 2019-08-18 the Raspberry pi firmware + u-boot takes ~18MiB
@@ -268,11 +277,11 @@ in
             echo "file sd-image $img" >> $out/nix-support/hydra-build-products
           fi
 
-          root_fs=${rootfsImage}
+          root_fs=${config.sdImage.rootFilesystemImage}
           ${lib.optionalString config.sdImage.compressImage ''
             root_fs=./root-fs.img
             echo "Decompressing rootfs image"
-            zstd -d --no-progress "${rootfsImage}" -o $root_fs
+            zstd -d --no-progress "${config.sdImage.rootFilesystemImage}" -o $root_fs
           ''}
 
           # Gap in front of the first partition, in MiB