Move all of NixOS to nixos/ in preparation of the repository merge

nixos/modules/tasks/filesystems/btrfs.nix

@@ -0,0 +1,47 @@
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+let
+
+  inInitrd = any (fs: fs == "btrfs") config.boot.initrd.supportedFilesystems;
+
+in
+
+{
+  config = mkIf (any (fs: fs == "btrfs") config.boot.supportedFilesystems) {
+
+    system.fsPackages = [ pkgs.btrfsProgs ];
+
+    boot.initrd.kernelModules = mkIf inInitrd [ "btrfs" "crc32c" ];
+
+    boot.initrd.extraUtilsCommands = mkIf inInitrd
+      ''
+        mkdir -p $out/bin
+        cp -v ${pkgs.btrfsProgs}/bin/btrfs $out/bin
+        ln -sv btrfs $out/bin/btrfsck
+        ln -sv btrfsck $out/bin/fsck.btrfs
+        # !!! Increases uncompressed initrd by 240k
+        cp -pv ${pkgs.zlib}/lib/libz.so* $out/lib
+        cp -pv ${pkgs.lzo}/lib/liblzo2.so* $out/lib
+      '';
+
+    boot.initrd.extraUtilsCommandsTest = mkIf inInitrd
+      ''
+        $out/bin/btrfs --version
+      '';
+
+    boot.initrd.postDeviceCommands = mkIf inInitrd
+      ''
+        btrfs device scan
+      '';
+
+    # !!! This is broken.  There should be a udev rule to do this when
+    # new devices are discovered.
+    jobs.udev.postStart =
+      ''
+        ${pkgs.btrfsProgs}/bin/btrfs device scan
+      '';
+
+  };
+}