Airplane Twin Motor & Differential Thrust

[AustinSaintAubin]

I have noticed the following issues with cleanflight & cleanflight-configurator:

• Lacking documentation &/or option to enable the second motor output for Airplane? (Custom Airplane with Differential Thrust #1370)
  • The image in the configurator indicates that two motors should be selectable.
• Lacking documentation &/or option to setup / configure Differential Thrust on the Flight Controller?
• CUSTOM AIRPLANE mix is missing from the Configurator (fixed/added in Development branch) but still seems to be unavailable in CLI options in firmware.
  • Documentation on how to configure CUSTOM AIRPLANE mixes is missing in the mixes documentation. I would push an example... but I need one myself because I still don't understand how to configure a custom mix for an airplane.

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Issue is linked to #1250 in cleanflight/cleanflight

[AustinSaintAubin]

As a side note, there might be an issue with the Servo mapping as displayed on the Motors tab on Cleanflight Configurator 0.66.0 Dev version. See #251