Add minimal btrfs support, as per bug #303529.

RAID0 btrfs volumes can be detected only if btrfsctl is started.
Since embedding extra executables in initramfs is already possible,
add minimal btrfs support inside linuxrc.

defaults/initrd.scripts

@@ -955,6 +955,11 @@ cdupdate() {
 	fi
 }
 
+setup_btrfsctl() {
+	# start BTRFS volume detection, if available
+	[ -x /sbin/btrfsctl ] && /sbin/btrfsctl -a
+}
+
 setup_md_device() {
 	local device
 

defaults/linuxrc

@@ -304,6 +304,9 @@ startiscsi
 # let iscsid settle - otherwise mounting the iSCSI-disk will fail (very rarely, though)
 sleep 1
 
+# Setup btrfs, see bug 303529
+setup_btrfsctl
+
 # Setup md device nodes if they dont exist
 setup_md_device