This lowers the MPU9250 sampling rate from 1kHz to 500 Hz.

The tests don't work properly on Snapdragon yet, so just report happy
for now. Otherwise users get messages such as "No Gyro found" which is
wrong.

`sdlog2 start` lead to a segfault.

Only free the logbuffer if you exit sdlog2. After disarming, just reset
the pointers. This way, we don't take any risks doing malloc and free.

The actual "double free" was caused by trying to free the perf counter
inside logbuffer_free.

This allows resetting of the logbuffer without having to free and malloc
again.

added descend into navigation using existing land helper, downstream logic handles ignoring position setpoints (despite being populated), tested with multirotor in gazebo

Timestamps didn't exist when commander was run on the Linux side.
However, commander is now always run on the QURT side, so the hack is no
longer needed.