[AVR] Send USB ZLP if required

[facchinm]

Fixes #5732 , "reverts" #4864

Tested with all the following problematic code on Linux, WinXp and Win10

https://github.com/NicoHood/HID/blob/master/examples/RawHID/RawHID/RawHID.ino (only on Linux)

void setup() {
  // put your setup code here, to run once:

}

void loop() {
  if (Serial) {
    if (Serial.available() > 0) {
      Serial.read();
      char buff[65] = "D_ENDPOINT(USB_ENDPOINT_IN (CDC_ENDPOINT_IN ),USB_ENDPOINT_TYPE";
      Serial.write(buff,64);
      Serial.flush();
    }
  }
}

void setup() {
  // put your setup code here, to run once:

}

void loop() {
  if (Serial) {
    if (Serial.available() > 0) {
      Serial.read();
      char buff[129] = "D_ENDPOINT(USB_ENDPOINT_IN (CDC_ENDPOINT_IN ),USB_ENDPOINT_TYPED_ENDPOINT(USB_ENDPOINT_IN (CDC_ENDPOINT_IN ),USB_ENDPOINT_TYPE";
      Serial.write(buff,128);
      Serial.flush();
    }
  }
}

void setup() {
  // put your setup code here, to run once:

}

void loop() {
  if (Serial) {
    if (Serial.read() == 'h'){
      Serial.write("Hello ");
      Serial.flush();
      Serial.write("World!\r\n");
      Serial.flush();
    }
  }
}

Comments are really appreciated (as the USB core is still really convoluted, I'm probably missing something important)

@NicoHood @sandeepmistry @matthijskooijman @embmicro

[cmaglie]

I think that this is the wrong way to handle ZLP since the buffer could be filled in multiple rounds, for example this sketch still fails after the patch:

void setup() { }
void loop() {
  if (Serial) {
    if (Serial.available() > 0) {
      Serial.read();
      char buff[65] = "<--3456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABC-->";
      Serial.write(buff,32);
      Serial.write(buff+32,32);
      Serial.flush();
    }
  }
}

Pushed another solution on the same branch

[ArduinoBot]

✅ Build completed.

Please test this code using one of the following:

⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-5802-BUILD-635-linux32.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-5802-BUILD-635-linux64.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-5802-BUILD-635-windows.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-5802-BUILD-635-macosx.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-5802-BUILD-635-linuxarm.tar.xz

ℹ️ The linuxarm build is still experimental and may not be always available.