Create a udev rule for /dev/root

In the installation CD, this ensures that systemd will see a proper
udev node for /dev/root.

Fixes #4399.

Borrowed from

http://lists.freedesktop.org/archives/systemd-devel/2014-July/020988.html

nixos/modules/system/boot/stage-1-init.sh

···
 @postMountCommands@
 
 
+# Emit a udev rule for /dev/root to prevent systemd from complaining.
+eval $(udevadm info --export --export-prefix=ROOT_ --device-id-of-file=$targetRoot || true)
+if [ "$ROOT_MAJOR" -a "$ROOT_MINOR" -a "$ROOT_MAJOR" != 0 ]; then
+    mkdir -p /run/udev/rules.d
+    echo 'ACTION=="add|change", SUBSYSTEM=="block", ENV{MAJOR}=="'$ROOT_MAJOR'", ENV{MINOR}=="'$ROOT_MINOR'", SYMLINK+="root"' > /run/udev/rules.d/61-dev-root-link.rules
+fi
+
+
 # Stop udevd.
 udevadm control --exit || true
 

nixos/modules/system/boot/stage-1.nix

···
       (isYes "BLK_DEV_INITRD")
     ];
 
-    # Prevent systemd from waiting for the /dev/root symlink.
-    systemd.units."dev-root.device".text = "";
-
     boot.initrd.supportedFilesystems = map (fs: fs.fsType) fileSystems;
 
   };