Add USB send timeout

[sandeepmistry]

This change adds a 70 millisecond timeout on USB sends.

Related to one of the problems in arduino-libraries/WiFi101#72 (comment). On Windows, if you open and close the Serial Monitor with a sketch that uses SerialUSB.println(...) the entire sketch will hang.

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b86_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 Add USB send timeout #152
5. Select one of the boards under SAMD Pull Request Add USB send timeout #152 in Tools->Board menu
6. Compile/Upload as usual

[sandeepmistry]

Hi @RamonRibes,

Could you please tryout this build with the following sketch. Then open and close the Serial Monitor in the IDE. The LED should continue to blink after you do so.

void setup() {
  Serial.begin(9600);
  Serial.println("hello");

  pinMode(6, OUTPUT);
}

void loop() {
  Serial.println("LED HIGH");
  digitalWrite(6, HIGH);
  delay(1000);

  Serial.println("LED LOW");
  digitalWrite(6, LOW);
  delay(1000);
}

[RamonRibes]

Test procedure:
-Arduino IDE 1.6.9 (new installation on a Windows 10 new PC).
-Loaded standard SAMD boards library.
-Selected MKR1000.
-Compiled&started your sketch (Serial monitor kept closed).
-The built-in led does as intended.
-After 12 minutes, i opened the serial monitor. Led is doing OK. Monitor shows messages.
-After 12 minutes, i closed the serial monitor. Led remains in HIGH state.

---

-Build Pull Request #152 as instructed.
-Selected MKR1000 from Pull Request #152.
-Compiled&started your sketch (Serial monitor kept closed).
-The built-in led does as intended.
-After 12 minutes, i opened the serial monitor. Led is doing OK. Monitor shows messages.
-After 12 minutes, i closed the serial monitor. Led remains switching on/off each second.
-After 12 minutes, i re-opened the serial monitor. Led is doing OK. Monitor shows messages again.
...so the patch is working OK!
Congratulations! :-)

[sandeepmistry]

@RamonRibes great news!

So there is a solution for 1/3 problems you are facing so far.

[sandeepmistry]

Closing this now in favour of #154.