I'll get this upstream shortly. The stock layout is, as you can see, rather awkward - there's a huge amount of wasted space, but freeing it will prevent rolling back to stock firmware without a full flash backup, and possibly also require relocating the MAC address in flash.
Apologies for the delay, as of 97da92b, kernel and rootfs partition sizes are automatically calculated instead of being fixed in size. A new snapshot has already been successfully built. That should be the end of this bug :)