Prevent full UART TX buffer from hanging processor

[sandeepmistry]

Similar to #260, but with additional checks.

When UART::write(b) is called and the TX buffer is full it will now manually call the DRE IRQ handler if:

• interrupts are currently disabled
  or
• the current IRQ (exception number) has a higher or equal priority than the SERCOM IRQ

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b167_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request Prevent full UART TX buffer from hanging processor #262
5. Select one of the boards under SAMD Pull Request Prevent full UART TX buffer from hanging processor #262 in Tools->Board menu
6. Compile/Upload as usual

[sandeepmistry]

Tested with the following sketch:

void setup() {
  Serial.begin(9600);
  while (!Serial);

  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(2, onPin2Change, CHANGE);
}

void loop() {
  Serial.println(millis());

  delay(1000);
}

void onPin2Change() {
  digitalWrite(LED_BUILTIN, digitalRead(2));


  Serial.println("onPin4Change1");
  Serial.println("onPin4Change2");
  Serial.println("onPin4Change3");
  Serial.println("onPin4Change4");
  Serial.println("onPin4Change5");
  Serial.println("onPin4Change6");
  Serial.println("onPin4Change7");
  Serial.println("onPin4Change8");
  Serial.println("onPin4Change9");
  Serial.println("onPin4Change0");
  Serial.println("onPin4Change1");
  Serial.println("onPin4Change2");
  Serial.println("onPin4Change3");
  Serial.println("onPin4Change4");
  Serial.println("onPin4Change5");
  Serial.println("onPin4Change6");
  Serial.println("onPin4Change7");
  Serial.println("onPin4Change8");
  Serial.println("onPin4Change9");
  Serial.println("onPin4Change0");

  Serial.println();
}

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b173_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request Prevent full UART TX buffer from hanging processor #262
5. Select one of the boards under SAMD Pull Request Prevent full UART TX buffer from hanging processor #262 in Tools->Board menu
6. Compile/Upload as usual