Allow the user to specify their serial port scenarios.

docs/Configuration.md

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+
+## Serial port functions and scenarios
+
+### Serial port scenarios
+
+```
+0   UNUSED
+1   MSP, CLI, TELEMETRY, GPS-PASTHROUGH
+2   GPS ONLY
+3   SERIAL-RX ONLY
+4   TELEMETRY ONLY
+5   MSP, CLI, GPS-PASTHROUGH
+6   CLI ONLY
+7   GPS-PASSTHROUGH ONLY
+8   MSP ONLY
+```
+
+### Contraints
+
+* There must always be a port available to use for MSP
+* There must always be a port available to use for CLI
+* To use a port for a function, the function's corresponding feature must be enabled first.
+e.g. to use GPS enable the GPS feature.
+* If the configuration is invalid the serial port configuration will reset to it's defaults and features may be disabled.
+
+### Examples
+
+All examples assume default configuration (via cli `defaults` command)
+
+a) GPS and TELEMETRY (when armed)
+
+- TELEMETRY,MSP,CLI,GPS PASSTHROUGH on UART1
+- GPS on UART2
+
+```
+feature TELEMETRY
+feature GPS
+save
+```
+
+b) SERIAL_RX and TELEMETRY (when armed)
+
+- TELEMETRY,MSP,CLI,GPS PASSTHROUGH on UART1
+- SERIAL_RX on UART2
+
+```
+feature TELEMETRY
+feature SERIAL_RX
+set serial_port_2_scenario = 3
+save
+```
+
+c) GPS and TELEMETRY via softserial
+
+- TELEMETRY,MSP,CLI,GPS PASSTHROUGH on UART1
+- GPS on UART2
+
+```
+feature TELEMETRY
+feature GPS
+feature SOFTSERIAL
+set serial_port_3_scenario = 4
+save
+```
+d) SERIAL_RX, GPS and TELEMETRY (when armed) MSP/CLI via softserial
+
+- GPS on UART1
+- SERIAL RX on UART2
+- TELEMETRY,MSP,CLI,GPS PASSTHROUGH on SOFTSERIAL1
+Note: PPM needed to disable parallel PWM which in turn allows SOFTSERIAL to be used.
+
+```
+feature PPM
+feature TELEMETRY
+feature GPS
+feature SERIALRX
+feature SOFTSERIAL
+set serial_port_1_scenario = 2
+set serial_port_2_scenario = 3
+set serial_port_3_scenario = 1
+set msp_baudrate = 19200
+set cli_baudrate = 19200
+set gps_passthrough_baudrate = 19200
+save
+```