Fix: No OpenDTU startup of CDC (like Pico or ESP32-S3) devices if no usb host is connected

Should also fix #826

platformio.ini

@@ -214,3 +214,4 @@ build_flags = ${env.build_flags}
     -DCMT_GPIO3=8
     -DCMT_SDIO=5
     -DARDUINO_USB_MODE=1
+    -DARDUINO_USB_CDC_ON_BOOT=1

src/main.cpp

@@ -26,8 +26,13 @@ void setup()
 {
     // Initialize serial output
     Serial.begin(SERIAL_BAUDRATE);
+#if ARDUINO_USB_CDC_ON_BOOT
+    Serial.setTxTimeoutMs(0);
+    delay(100);
+#else
     while (!Serial)
         yield();
+#endif
     MessageOutput.println();
     MessageOutput.println("Starting OpenDTU");