TX16s buddy box (wireless master/trainer)

I wanted a wireless buddy box for my Radiomaster TX16s

Solutions does already exist, Radiomaster is selling "Sbus Wireless Trainer Modules" but they need SBUS compatible receivers to be added. So won't work with old PPM radios.
Furthermore, their modules need to be linked (soldered with) mini receiver to work. And they are "one way" modules (trainer to slave) and finally quite expensive...

So decided to build mine with the following specifications:

- cheap (two fully working modules for less than 20$)
- auto configuration as master or slave or vice versa
- auto binding between two modules
- fully hidden into the "external module" compartment (no dangling wire outside the radio, no cover modification ))
- no modification into the TX16s
- no firmware modification of your TX16s (stock EdgeTx firmware)
- high speed wireless communication with about 20m range
- automatic On/Off of the buddy boxes via EdgeTx
- compatible 16 channels SBUS and PPM signals
- extensible to further options (t