Expose libmodbus's flush command

* Expose libmodbus's flush command to ModbusRTUServer API

Author: pomplesiegel

Diff for: src/ModbusRTUServer.cpp

@@ -70,4 +70,9 @@ int ModbusRTUServerClass::poll()
   return 0;
 }
 
+int ModbusRTUServerClass::flush()
+{
+  return modbus_flush(_mb);
+}
+
 ModbusRTUServerClass ModbusRTUServer;

Diff for: src/ModbusRTUServer.h

@@ -46,6 +46,11 @@ class ModbusRTUServerClass : public ModbusServer {
    */
   virtual int poll();
 
+  /**
+   * Flush modbus
+   */
+  virtual int flush();
+
 private:
   RS485Class* _rs485 = &RS485;
 };