feat: more flexible ADC inputs

[raphaelcoeffic]

Summary of changes:

• use FLEX instead of POT ,AXIS, and MOUSE
• add support for configuring ADC input as SWITCH
• support for ADC inversion in UI

@elecpower there is yet another change in YAML format (for the better likely)

This PR introduces the following YAML changes:

• potsConfig has additional types: switches, axis_x and axis_y.
• potsConfig now supports inversion with inv (0 or 1).

potsConfig:
   EXT1:
      type: switch
      inv: 0
      name: ""
   EXT2:
      type: switch
      inv: 0
      name: ""
   EXT3:
      type: axis_x
      inv: 0
      name: ""
   EXT4:
      type: axis_y
      inv: 0
      name: ""

• a new block called flexSwitches defines up to 2 additional on targets supporting this (only TX16 for now):
  Please note: flexConfig must be placed after potsConfig. The loader will verify that the channel (analog source) is indeed configured as switch.

flexSwitches: 
   FL1:
      channel: EXT1
   FL2:
      channel: EXT2

• up to 2 new switches called FL1 and FL2

[3djc]

We wont commit to it, but this is likely the last of the 'brace for impact' PR for 2.10 (maybe)

[JyeSmith]

Yay!

[raphaelcoeffic]

@JyeSmith did you try it out?

[JyeSmith]

Not yet, but I'll put it on the list. I have a tx16 that will need to be unmodified to test.

[pfeerick]

Regression tested on TX16S, TX12MKII and X9D+2019 hardware, no startup or calibration checksum failures.

[philmoz]

This PR breaks existing models that use the 6POS switch as a source.
The 6POS source string in the YAML file is not recognised and ends up as no source.

False alarm - needed to do a clean build.