kernel-plus 2.6.32.61 r6 focuses on btrfs. It has been enabled as [Y] in the kernel. Note that the disk format as used in the kernel probably matches 2.6.34 - ubiboot definitely has not been updated to support reading from this version of the btrfs disk format. Play with /home or something. Meanwhile, or if you're feeling adventurous, load zImage from somewhere other than the Harmattan rootfs (check ubiboot.conf)