[LUA API] new function tele.isOn()

[JimB40]

this refers #777

In current implementation to check if Telemetry data is coming we have to:

1. getRSSI() and check against 0.
2. getValue('sensorName') against 0.

There should be simple function tele.isOn() returning true/false if telemetry stream is received to simplify and unify this checking if telemetry data is actually received

Caveat: With newest protocol implementation (Ghost, CRSF, ELRS) there are some sensors that are actually send from Tx rather then from Rx. I call it 'off-line' telemetry. Example in Ghost protocol all VTX sensors are stored in Tx and available (sending data) even Rx is not powered.

[pfeerick]

Hm... is that as in some telemetry is actually from the module, not the receiver?

[JimB40]

Yes it is :)