System feedback message logging

This commit enables logging of system text feedback in a file. As its rate-limited within MAVLink, this cannot lead to excessive file IO. Every message that is sent is logged on the microSD card.

This is useful for event feedback of the system and debugging user setups, as it gives clear indications what is wrong with the system.

It also resets the performance counters on arming, as some of the large max counts / values stem from setup procedures - but we're interested in timing overruns during flight, not during prep.