Function added to detect baudrate (#4978)

* Function added to detect baudrate

* Added uart_start_detect_baudrate, detectBaudrate() wrappers for HardwareSerial and an example usage SerialDetectBaudrate.ino

* Some layout changes to pass Travis tests

* Some more nitty-gritty layout changes to pass Travis tests

* Some even more nitty-gritty layout changes to pass Travis tests

* renamed one function to testBaudrate() and updated doc/reference.rst

* Minor updates to doc/reference.rst

* New lines added

libraries/esp8266/examples/SerialDetectBaudrate/SerialDetectBaudrate.ino

@@ -0,0 +1,34 @@
+#define TIMEOUT (10000UL)     // Maximum time to wait for serial activity to start
+
+void setup() {
+  // put your setup code here, to run once:
+
+  Serial.begin(115200);
+
+  // Serial.detectBaudrate() may also be called before Serial.begin()
+  // There must be activity on the serial port for the baudrate to be detected
+  unsigned long detectedBaudrate = Serial.detectBaudrate(TIMEOUT);
+
+  if (detectedBaudrate) {
+    Serial.printf("\nDetected baudrate is %lu, switching to that baudrate now...\n", detectedBaudrate);
+
+    // Wait for printf to finish
+    while (Serial.availableForWrite() != UART_TX_FIFO_SIZE) {
+      yield();
+    }
+
+    // Clear Tx buffer to avoid extra characters being printed
+    Serial.flush();
+
+    // After this, any writing to Serial will print gibberish on the serial monitor if the baudrate doesn't match
+    Serial.begin(detectedBaudrate);
+  } else {
+    Serial.println("\nNothing detected");
+  }
+}
+
+void loop() {
+  // put your main code here, to run repeatedly:
+
+}
+