comver Mavlink to passthrough

This can do translation from mavlink to passthru

it works in 3 modes, as I understand it's useful:

1. if your FC does not support s.port ot passthru

2. you can get mavlink through external telemetry, and want to feed into your radio.

3. relay mode, some how complicated for me as a setup.

The passthru is at 57600bps, and quite efficient as it use bit map:

For telemetry, there were esp dronebridge using wifi, and this one using espNow for better range: https://discuss.ardupilot.org/t/simple-esp-telemetry-serial-bridges/75536/32?page=2

also this one:

combine mav2passthu and espnow as tx module.

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