partial btrfs support

doesn't actually know how to make the filesystem, just
kernel config and accept it as a valid option

modules/outputs/btrfs.nix

@@ -0,0 +1,37 @@
+{
+  config
+, pkgs
+, lib
+, ...
+}:
+let
+  inherit (lib) mkIf mkOption types;
+  o = config.system.outputs;
+in
+{
+  imports = [
+    ./initramfs.nix
+  ];
+  config = mkIf (config.rootfsType == "btrfs") {
+    kernel.config = {
+      BTRFS_FS = "y";
+    };
+    boot.initramfs.enable = true;
+    system.outputs = {
+      rootfs =
+        let
+          inherit (pkgs.pkgsBuildBuild) runCommand e2fsprogs;
+        in runCommand "mkfs.btrfs" {
+          depsBuildBuild = [ e2fsprogs ];
+        } ''
+          tree=${o.bootablerootdir}
+          size=$(du -s --apparent-size --block-size 1024 $tree |cut -f1)
+          # add 25% for filesystem overhead
+          size=$(( 5 * $size / 4))
+          dd if=/dev/zero of=$out bs=1024 count=$size
+          echo "not implemented" ; exit 1
+          # mke2fs -t ext4 -j -d $tree $out
+        '';
+    };
+  };
+}