Replace ancient 64-btrfs.rules with newer systemd rules

This solves the failure to boot when the root fs is on a btrfs handled
raid set-up. The reason it fails is typically because one or both disks
failure are not initialised before the kernel wants to mount root.

Solves issues #1329 and #1980

conf/64-btrfs.rules

@@ -1 +1,17 @@
-RUN+="/bin/mknod /dev/btrfs-control c 10 234"
+# do not edit this file, it will be overwritten on update
+
+SUBSYSTEM!="block", GOTO="btrfs_end"
+ACTION=="remove", GOTO="btrfs_end"
+ENV{ID_FS_TYPE}!="btrfs", GOTO="btrfs_end"
+ENV{SYSTEMD_READY}=="0", GOTO="btrfs_end"
+
+# let the kernel know about this btrfs filesystem, and check if it is complete
+IMPORT{builtin}="btrfs ready $devnode"
+
+# mark the device as not ready to be used by the system
+ENV{ID_BTRFS_READY}=="0", ENV{SYSTEMD_READY}="0"
+
+# reconsider pending devices in case when multidevice volume awaits
+ENV{ID_BTRFS_READY}=="1", RUN+="@rootbindir@/udevadm trigger -s block -p ID_BTRFS_READY=0"
+
+LABEL="btrfs_end"