SmartAudio Lite compatibility: Send 0x00 bit(s) (always) even with Soft Serial.

[fujin]

I've been sitting on this for a while. Basically, I noticed that with the my AIKONF4's which expose a 'vtx' control pad on SoftSerial with SmartAudio Lite on the TBS Unify Nano (genuine), that it would sit in autobaud state.

I hacked this up to send the extra 0x00's all the flippin time, and it seems to work enough to be usable. As the configurable UART support we have discussed (inversion, pull-down/pull-up, baud, stopbits, blahblah) internally has not yet surfaced.. here's a hack instead.

I'm personally not in favour of a 10k hard pull down workaround - I have always run the patch to avoid it - and the latest revision of TBS SA spec document calls for the host MCU to pull the line low if the hard pull down can not be 100% guaranteed. One could make the argument that the host should also be SA protocol-standards compliant to best effort: we cannot ensure the hard-pull down, i.e. we must pull the line low (as even if a hard pull-down existed, it would effectively be idempotent) in all configurations.

The major downside of this change is that it obviously 💇‍♂️ supports their license non-compliant competitor e.g. early AKK/Mach2 drama-llama variants.

[mikeller]

We've held off on this for some time now, while trying to get traction for a unified way to set UART parameters, but this is not likely to happen in 4.0, so I think the right course of action is to add support for SmartAudio lite now, and make the pull down configurable once we get the new UART parameter implementation.

Re support for AKK VTX: My concern at the time was that, since the pull down was not mandated in the then current official SmartAudio spec, TBS would at some point release VTX models that did not work with this. Since they are now mandating the pulldown for the SmartAudio lite this is not an issue any more.

[blckmn]

Merging these as we wait for the completion of the UART custom settings implementation..

[mikeller]

Fixes #5329.