Fixes an issue with STM32F4 flow control when RTS&CTS are not connected

With STM32F4 targets UARTSerial.set_flow_control() must be called with both
pins(RTS&CTS) enabled. If only one is given assertion will fail. This
fix will help to circumvent the issue with STM32F4 when disabling flow
control and no pins are given. This won't solve the issue with STM32F4
HAL itself if only one pin is given.

Author: Veijo Pesonen

ESP8266/ESP8266.cpp

@@ -112,12 +112,16 @@ struct ESP8266::fw_at_version ESP8266::at_version()
 
 bool ESP8266::stop_uart_hw_flow_ctrl(void)
 {
-    // Stop board's flow control
-    _serial.set_flow_control(SerialBase::Disabled, _serial_rts, _serial_cts);
+    bool done = true;
+
+    if (_serial_rts != NC || _serial_cts != NC) {
+        // Stop board's flow control
+        _serial.set_flow_control(SerialBase::Disabled, _serial_rts, _serial_cts);
 
-    // Stop ESP8266's flow control
-    bool done = _parser.send("AT+UART_CUR=%u,8,1,0,0", ESP8266_DEFAULT_BAUD_RATE)
-        && _parser.recv("OK\n");
+        // Stop ESP8266's flow control
+        done = _parser.send("AT+UART_CUR=%u,8,1,0,0", ESP8266_DEFAULT_BAUD_RATE)
+            && _parser.recv("OK\n");
+    }
 
     return done;
 }
@@ -137,10 +141,12 @@ bool ESP8266::start_uart_hw_flow_ctrl(void)
     } else if (_serial_rts != NC) {
         _serial.set_flow_control(SerialBase::RTS, _serial_rts, NC);
 
+        // Enable ESP8266's CTS pin
         done = _parser.send("AT+UART_CUR=%u,8,1,0,2", ESP8266_DEFAULT_BAUD_RATE)
             && _parser.recv("OK\n");
 
     } else if (_serial_cts != NC) {
+        // Enable ESP8266's RTS pin
         done = _parser.send("AT+UART_CUR=%u,8,1,0,1", ESP8266_DEFAULT_BAUD_RATE)
             && _parser.recv("OK\n");