Merge pull request arduino#123 from KurtE/no_usb_no_hang

If sketch is not connected to serial - dump serial output

Author: facchinm

cores/arduino/SerialUSB.h

@@ -18,6 +18,8 @@ class SerialUSB_ : public ZephyrSerial {
 	void begin(unsigned long baudrate) { begin(baudrate, SERIAL_8N1); }
 
 	operator bool() override;
+	size_t write(const uint8_t *buffer, size_t size) override;
+	void flush() override;
 
 protected:
 	uint32_t dtr = 0;

cores/arduino/USB.cpp

@@ -124,5 +124,16 @@ arduino::SerialUSB_::operator bool() {
     return dtr;
 }
 
+
+size_t  arduino::SerialUSB_::write(const uint8_t *buffer, size_t size) {
+    if (!Serial) return 0;
+    return arduino::ZephyrSerial::write(buffer, size);
+}
+
+void arduino::SerialUSB_::flush() {
+    if (!Serial) return;
+    arduino::ZephyrSerial::flush();
+}
+
 arduino::SerialUSB_ Serial(usb_dev);
 #endif