Update Serial.h

Renamed tx_done to tx_empty
Created tx_complete

Author: Hsubtnarg

cores/arduino/Serial.h

@@ -78,7 +78,8 @@ class UART : public arduino::HardwareSerial {
     arduino::SafeRingBufferN<SERIAL_BUFFER_SIZE> rxBuffer;
     arduino::SafeRingBufferN<SERIAL_BUFFER_SIZE> txBuffer;
 
-    volatile bool tx_done;
+    volatile bool tx_empty;
+    volatile bool tx_complete;
 
     sci_uart_instance_ctrl_t  uart_ctrl;
     uart_cfg_t                uart_cfg;