nixos/installer: add new options to customize sd-card image creation (#450774)

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

@@ -21,7 +21,7 @@
 with lib;
 
 let
-  rootfsImage = pkgs.callPackage ../../../lib/make-ext4-fs.nix (
+  rootfsImage = pkgs.callPackage config.sdImage.rootFilesystemCreator (
     {
       inherit (config.sdImage) storePaths;
       compressImage = config.sdImage.compressImage;
@@ -128,6 +128,29 @@ in
       '';
     };
 
+    rootFilesystemCreator = mkOption {
+      type = types.oneOf [
+        types.package
+        types.path
+      ];
+      default = ../../../lib/make-ext4-fs.nix;
+      example = ''
+        nixpkgs/nixos/lib/make-btrfs-fs.nix
+      '';
+      description = ''
+        The filesystem creator used for the root partition.
+      '';
+    };
+
+    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
@@ -254,11 +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