Due to limited flash space

This is the quickest way to achieve the desired result.
Long-term we might do something else.

To get it field-tested. This can be reverted for a release...

- mission logs are stored in a separate directory mission_log
- It's disabled by default
- Does not increase RAM usage if disabled (if enabled, only 300 bytes)
- Log rotate does not apply to the mission logs

Some message formats are longer than the 300 bytes. We can split the writes
because we have to wait until they are written anyway.

256 subscriptions are enough for now.
Reduces RAM usage by 300 bytes.

Not used yet, it should not affect anything, except for slight RAM
increase.

Previously the formats of all known uorb messages were written.

- reduces header size by about 13KB
- reduce ulog_message_format_s size to reduce required stack size.
  Largest message format is about 1000 bytes.

Previously the acceptance radius was 0, so the FlightTaskAutoLine was
randomly changing yaw and sometimes going into a random direction.
There is still something else wrong in there, but avoids the reposition
problem.

If the filter was disabled, the apply() would always return 0.