OutSystemID should also be allowed as 0

[sanderux]

I'm writing a system that broadcasts adsb to all systems. TargetSystemID 0 should be allowed per mavlink standards.

This line does not allow OutSystemID to be set to 0 https://github.com/bluenviron/gomavlib/blob/c2f6b02b4fd10900c8526a8ef0a792f49bb8d3db/pkg/frame/writer.go#L52

@aler9 fyi

[sanderux]

See here: https://github.com/mavlink/qgroundcontrol/blob/master/src/Vehicle/Vehicle.cc#L564

[aler9]

Hello @sanderux, the OutSystemID field is unrelated from the TargetSystem field - the former is in the frame and identifies the source, while the latter is in the message and identifies the destination. The library doesn't prevent in any way using a TargetSystem equal to zero:

node.WriteMessageAll(common.MessageLoggingData{
    TargetSystem: 0,
    TargetComponent: 0,
    ...
})