sdlog2: move main state, fix formatting, fix bug

Since the vehicle_status topic has been split up, we can't publish it
together with the the split cpuload and commander_state topics.

Therefore, the log field STAT.MainState will change to COMM.MainState
because it is only the internal commander state. Important to the
outside is STAT.NavState.

Likewise, the log field STAT.Load becomes LOAD.CPU.