Add MATEKF411 target with one full duplex softserial: MATEKF411_FD_SFTSRL

[shellixyz]

Closes #4611

[leeph]

Curious, I was literally just thinking this would be useful, I hate not having a Bluetooth module in my F411-Wing planes for remote configurator.

However I tend to use the LED pin for lights (obviously) - but for flying wings, would it be possible to have the FD softserial on the two redundant servo signal outputs?

Obviously this would only be of use for flying wings with 2 servos, but would still be useful.

Either that or ST1 pin and just one of the servo signals, leaving three servos for the mixer?

[shellixyz]

@leeph The issue with using PWM outputs for softserial is that it is not possible to use the same timer for both softserial and PWM so since the following groups of outputs are using the same timer if you use one of them for SS you can't use the other ones for anything else than the same SS: 1-2, 2-3, 4-5-6-7. If you want to use the LED pin and are using less then 3 servos you can put a SS on S2-S3

[expipiplus1]

@shellixyz Would you be able to clarify what you mean about the timer groups? I couldn't quite match the groups you mentioned "1-2, 2-3, 4-5-6-7" with what I'm seeing in the F411's target.c. It seems like the groups there are: "1-2, 3-4, 5-6-7". In this case (assuming S7 isn't being used) S5 and S6 seem like a good pair to use for a softserial port.