Add baudRate function (#651)

usefull for ESP8266 compatibility

cores/esp32/HardwareSerial.cpp

@@ -89,7 +89,11 @@ size_t HardwareSerial::write(const uint8_t *buffer, size_t size)
     uartWriteBuf(_uart, buffer, size);
     return size;
 }
+uint32_t  HardwareSerial::baudRate()
 
+{
+	return uartGetBaudRate(_uart);
+}
 HardwareSerial::operator bool() const
 {
     return true;

cores/esp32/HardwareSerial.h

@@ -66,6 +66,7 @@ public:
     {
         return write((uint8_t) n);
     }
+    uint32_t baudRate();
     operator bool() const;
 
     void setDebugOutput(bool);