feat(serial): add timeout support to flush()

[fpistm]

when called by end().
Default, no timeout (0). When called by end(), default timeout is
TX_TIMEOUT which can be redefined if needed.

Fixes #2122.

Tested with below sketch on a Nucleo-WL55JC1.
Serial1 is instantiate with CTS/RTS. With original code, it wait forever while with this PR it continue after TIMEOUT_TX expiration.

HardwareSerial Serial1(PB7, PB6, PB3, PB4);
uint32_t baud_val[] = {
  1200,
  2400,
  4800,
  9600,
  115200,
  250000
};
// the setup routine runs once when you press reset:
void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
}

// the loop routine runs over and over again forever:
void loop() {
  for (uint8_t idx = 0; idx < 6; idx++) {
    Serial1.end();
    Serial.printf("Test %u\n", baud_val[idx]);
    Serial1.begin(baud_val[idx]);
    Serial1.println("ATE0");
  }
  delay(100);
}

Output

09:20:52.421 -> Test 1200
09:20:53.419 -> Test 2400
09:20:54.384 -> Test 4800
09:20:55.383 -> Test 9600
09:20:56.380 -> Test 115200
09:20:57.379 -> Test 250000
09:20:58.473 -> Test 1200
09:20:59.471 -> Test 2400
09:21:00.470 -> Test 4800
09:21:01.465 -> Test 9600