Document valid usage of lcd_goto_menu(), wrap lcd_second_serial_set() by HAS_SECOND_SERIAL_PORT macro.

Firmware/ultralcd.cpp

@@ -3608,13 +3608,15 @@ void lcd_temp_calibration_set() {
 	lcd_goto_menu(lcd_settings_menu, 10);
 }
 
+#ifdef HAS_SECOND_SERIAL_PORT
 void lcd_second_serial_set() {
 	if(selectedSerialPort == 1) selectedSerialPort = 0;
 	else selectedSerialPort = 1;
 	eeprom_update_byte((unsigned char *)EEPROM_SECOND_SERIAL_ACTIVE, selectedSerialPort);
 	MYSERIAL.begin(BAUDRATE);
-	lcd_goto_menu(lcd_settings_menu, 11);
+	lcd_goto_menu(lcd_settings_menu, 11);//doesn't break menuStack
 }
+#endif //HAS_SECOND_SERIAL_PORT
 
 void lcd_calibrate_pinda() {
 	enquecommand_P(PSTR("G76"));