Fixedwing mem handling

@sjwilks This makes the memory fragmentation deterministic - previously you could have a "good" or "bad" boot timing with the largest mem block being 8K or 22K (free mem always 22K, but if its fragmented, that might still not be enough for MAVLink).

This likely also helps with boot stability on FMUv1, in addition to the generally increased stack size.

Code review appreciated.