Support common multiplexer

[MobiFlight-Admin]

There are multiplexer modules available for Arduino that are used by some sim builders and they could use mobiflight if it supported these multiplexer.

The 74Hc4067 would be a good choice because these are used by SimVim users and they are easily available on eBay and aliexpress for cheap money with a breakout board .

https://www.instructables.com/Tutorial-74HC4067-16-Channel-Analog-Multiplexer-De/?amp_page=true

[Issue created by DocMoebiuz: 2020-10-08]

[neilenns]

This would be amazing :)

[MobiFlight-Admin]

I updated the description a little bit

[Matze2010]

I would like to come up with a proof of concept for adding multiplexers. However, from what I understand so far, mobiflight keeps tracks of all the assigned pins and in the UI offers only free pins available. This practice would have to change when adding multiplexer support, as the multiplexer pins are shared over several modules. This would be a rather big change in the mobiflight source and UI, so I think it would make sense to discuss possible solutions first and before starting an implementation.

[neilenns]

@Matze2010 lots of off and on discussion about this work in discord lately. I suggest joining the chat in #development before doing anything, Sebastian has a specific chip he wants to support for this that would work with the existing pin model (doesn't use I2C).