Instead of using 255.255.255.255 as the broadcast address, go through
the network interfaces and and try to use a broadcast in the local
network such as 192.168.1.255.

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.