Re-factor UART pin assignments. #70
Description
The situation
CRSF for Arduino defaults to Serial1 for UART.
For most targets, this means pins 0 and 1 for Rx and Tx, respectively.
This is okay if you're using Arduino or Adafruit Metro or Adafruit Feather targets, however this is problematic for targets that don't use pins 0 and 1 for UART - Especially Seeed Xiao and Adafruit QtPy boards that are built around ESP32 chipsets.
Possible solutions
Use hardware-dependent default pin-outs instead of arbitrary pin-outs
Looking at the pins_arduino.h header in each target's variants subfolder, there are references to RX and TX which set the default pin-outs for the first UART port. CRSF for Arduino could take advantage of this instead of defaulting to pins 0 and 1.
Use arbitrary UART port and pin assignments
This could effectively grant you the ability to tell CRSF for Arduino what particular pins your receiver is connected to and what UART port the pins are using. IE Customisable port and pin assignments.
The limiting factor here is the number of UART ports is hardware-dependent. Some targets have two possible UART ports, while others may have up to six possible UART configurations.
For targets that have only one UART port, you will not be able to take advantage of customisable UART pin-outs.
This is a hardware limitation that is outside the scope of CRSF for Arduino.