Add configurable aux LED channel

[SammysHP]

For an older version of Anduril I started to implement a new channel that allows the user to select the aux color:

https://github.com/SammysHP/flashlight-firmware/compare/multi-channel...mc-auxmix-channel

Instead of having seven additional channels, one for each color, there would be only one channel with 3H changing the color. It also included some changes to allow to overwrite the number of steps, so that it doesn't conflict with tint ramping steps or alignment.

In the current state it cannot be applied and it could also need some improvements like:

• maintain the possibility to use a specific color for blinking and menus
• add software PWM for brightness ramping
• support an extended color model (like RGB main emitters)

[SiteRelEnby]

The main problem I see with this one is how it affects blinky modes (would need code changes). I use the aux channels on some lights as a ramp mode channel, but I don't want the blinky mode colour to change every time I do.

Would be possible to just add a new EEPROM var for something like aux_channel_blink_arg and save it independently when the user does a 3C in battcheck mode, I guess (or just 3C to aux mode, then 3H to adjust arg which is just saved separately, might be simpler).