feat (uart): implement proper operator bool()

[patricklaf]

This PR implements the HardwareSerial bool() operator.

Currently, the core blocks if there is a write to a closed HardwareSerial instance. The tx buffer will rapidly be full and the core waits forever for free space.

The bool() operator is a good way to check if the HardwareSerial object is opened and ready to transmit data but in the current implementation, it allways returns true.

The PR implements a complete bool() operator:

• Returns true when the HardwareSerial object is ready.
• Returns false when the HardwareSerial object is not ready.

Sample code:

Serial.begin();
if (Serial) {
  Serial.println("Serial is opened");
  Serial.flush();
}
Serial.end();
if (Serial) {
  Serial.println("Serial is closed. Never be printed");
}