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In order to program the BLHeli ESC via a telemetry connection, the following things have to be done:
SERVO_BLH_AUTO 1 -> enable
SERVO_BLH_MASK 4 -> activate for servo channel 3 (00100... = 4)
SERVO_BLH_OTYPE 6 -> DSHot600
SERVO_BLH_PORT 1 -> (optional) redirect to Serial1 if needed (RFD Telemetry Port / otherwise USB)
SERVO_DSHOT_ESC 1 -> configure DShot type to BLHeli