feat(ACCESS): Configure AUX serial port as "External module" (#2562)

* Enable RX DMA on external module USART

* feat(ACCESS): configure AUX serial port as "External module" (#2562)

This allows configuring the external access mod on runtime rather than by compilation option.

* fix: fetch external module driver

radio/src/pulses/afhds3.cpp

@@ -864,7 +864,9 @@ void applyModelConfig(uint8_t module)
 static void* initExternal(uint8_t module)
 {
 #if defined(AFHDS3_EXT_UART)
-  const etx_serial_driver_t* drv = &ExtmoduleSerialDriver;
+  auto drv = extmoduleGetSerialPort();
+  if (!drv) return nullptr;
+
   uint16_t period = AFHDS3_UART_COMMAND_TIMEOUT * 1000 /* us */;
 #else
   const etx_serial_driver_t* drv = nullptr;