lib: Create make-btrfs-fs library to match make-ext4-fs.

I wrote this up when I was playing with a btrfs sd card, and
figured that it would be useful for others.

nixos/lib/make-btrfs-fs.nix

@@ -0,0 +1,45 @@
+# Builds an btrfs image containing a populated /nix/store with the closure
+# of store paths passed in the storePaths parameter, in addition to the
+# contents of a directory that can be populated with commands. The
+# generated image is sized to only fit its contents, with the expectation
+# that a script resizes the filesystem at boot time.
+{ pkgs
+, lib
+# List of derivations to be included
+, storePaths
+# Shell commands to populate the ./files directory.
+# All files in that directory are copied to the root of the FS.
+, populateImageCommands ? ""
+, volumeLabel
+, uuid ? "44444444-4444-4444-8888-888888888888"
+, btrfs-progs
+}:
+
+let
+  sdClosureInfo = pkgs.buildPackages.closureInfo { rootPaths = storePaths; };
+in
+pkgs.stdenv.mkDerivation {
+  name = "btrfs-fs.img";
+
+  nativeBuildInputs = [ btrfs-progs ];
+
+  buildCommand =
+    ''
+      set -x
+      (
+          mkdir -p ./files
+          ${populateImageCommands}
+      )
+
+      mkdir -p ./files/nix/store
+      cp ${sdClosureInfo}/registration ./files/nix-path-registration
+
+      # Add the closures of the top-level store objects.
+      for p in $(cat ${sdClosureInfo}/store-paths); do
+        echo cp -r $p "./files/nix/store"
+      done
+
+      touch $out
+      mkfs.btrfs -L ${volumeLabel} -U ${uuid} -r ./files --shrink $out
+    '';
+}