Add serial_flush function

serial_flush() allows flushing the serial buffer from anywhere in Epicardium.

• When run from thread mode it will flush to UART, CDC-ACM and BLE. This is similar to what the serial task would do once it is rescheduled.
• When run inside an exception handler, it will only flush to UART because CDC-ACM and BLE cannot be flushed from an ISR. Note that characters flushed this way will never appear on the other outputs, even if the serial task is scheduled at some point afterwards.

The main use of this function is to ensure output of messages even in cases of critical failures.

Closes #164 (closed).

epicardium/modules/modules.h

@@ -29,6 +29,7 @@ void return_to_menu(void);
 void serial_init();
 void vSerialTask(void *pvParameters);
 void serial_enqueue_char(char chr);
+void serial_flush(void);
 extern TaskHandle_t serial_task_id;
 
 // For the eSetBit xTaskNotify task semaphore trigger