Append multithreding support to HardwareSerial

This fix will allow use HardwareSerial driver in simple multithreading applications.

Author: slavaza

cores/arduino/HardwareSerial.cpp

@@ -261,7 +261,8 @@ size_t HardwareSerial::write(uint8_t c)
       if(bit_is_set(*_ucsra, UDRE0))
 	_tx_udr_empty_irq();
     } else {
-      // nop, the interrupt handler will free up space for us
+	// share CPU while buffer free waiting
+	yield();
     }
   }