support "rssi_src_frame_errors"

[laurentopia]

Current Behavior

Rssi can only be gained by consuming a channel or connecting a rssi pad

Desired Behavior

count the lost packets like in betaflight = no consumed channel, no difficult setup and a pretty good way to indicate signal quality

Who does this impact? Who is this for?

everyone who does long range ;-)

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[digitalentity]

Implemented in #4941