WIP: broadcast target for rx

[landswellsong]

Hi,
Made this change for myself, but may be useful elsewhere. Allows to specify broadcast addresses for connect:// at rx side. Ideally we may want to check if the address is indeed a broadcast, but mavlink-router doesn't bother to do that so I'm not sure. Maybe it will fail if running as unprivileged user though.

[svpcom]

@landswellsong Why not use multicast 224.0.0.1 address instead of broadcast? It doesn't require any additional permissions

[landswellsong]

@svpcom great alternative, didn't think of that. But I have options of some SW on an android tablet that's not aware of multicast, I'm not sure it can join the multicast group normally without too much rewrite.

How about I add broadcasting as an option so it's only set if you explicitly mention it in the config?

[svpcom]

@landswellsong 224.0.0.1 multicast group is "local network" group and all hosts are members of it by default. It already used in wfb-ng video broadcasting example.