Add ring buffer for serial TX

[nseidle]

This PR makes Serial TX writes and prints interrupt driven rather than a blocking serial transfer one byte at a time. The _tx_buffer was there, I just implemented it. This reduces the time spent on Serial.prints significantly.

Here is example output running at 9600bps without this PR:

And here is example output with this PR:

Note the 66ms is because the TX ring buffer is 256bytes and myString is 293 so we have to wait for the UART to open up. If we increase the size of the ring buffer, this value goes to 0.

Also availableForWrite() method is now implemented.

Also in this PR is a patch for the HAL to make am_hal_uart_flags_get() work.

Here is my test sketch:

void setup() {
  Serial.begin(9600);
  delay(10);
  Serial.println("Serial buffer testing");

  String myString = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque eget ligula laoreet, faucibus felis sit amet, cursus mi. Praesent congue tristique congue. Pellentesque feugiat diam volutpat eros consectetur dapibus. Nullam lobortis enim at rutrum maximus. Sed lobortis purus in sagittis.";

  long myTimer = millis();
  Serial.println(myString);
  long delta = millis() - myTimer;
  
  Serial.println("Length: " + (String)myString.length());
  Serial.println("Time to send myString: " + (String)delta);
  Serial.println("Time to send with blocking method (calculated): " + (String)(myString.length() * (1000.0/960)));
  Serial.print("Available space in TX buffer: ");
  Serial.println(Serial.availableForWrite());
}

void loop() {

}

[oclyke]

Nice! Thanks for the excellent work. I just modified whitespace to make the changes to the HAL more obvious in the combined diff.