Allow sending multiple commands in one serial transmission

E.g. 
"1;2;1;0;2;1\n1;11;1;0;2;1\n1;1;1;0;2;0\n1;2;1;0;2;0\n"

libraries/MySensors/core/MyGatewayTransportSerial.cpp

@@ -54,6 +54,7 @@ bool gatewayTransportAvailable() {
 			if (inChar == '\n') {
 				_serialInputString[_serialInputPos] = 0;
 				available = true;
+				return true;
 			} else {
 				// add it to the inputString:
 				_serialInputString[_serialInputPos] = inChar;