define Serial as SerialTinyUSB if USE_TINYUSB is defined

Author: hathach

examples/CDC/cdc_multi/.feather_esp32s2.test.skip

@@ -0,0 +1,6 @@
+feather_esp32s2
+feather_esp32s3
+funhouse
+magtag
+metroesp32s2
+pico_rp2040_tinyusb_host

examples/CDC/cdc_multi/.feather_esp32s3.test.skip

@@ -93,15 +93,11 @@ class Adafruit_USBD_CDC : public Stream, public Adafruit_USBD_Interface {
   bool isValid(void) { return _instance != INVALID_INSTANCE; }
 };
 
-// "Serial" is used with TinyUSB CDC
-#if defined(USE_TINYUSB)
-extern Adafruit_USBD_CDC Serial;
-#define SerialTinyUSB Serial
-#endif
-
-// Serial is probably used with HW Uart
-#ifndef SerialTinyUSB
 extern Adafruit_USBD_CDC SerialTinyUSB;
+
+// Built-in support "Serial" is used with TinyUSB CDC
+#if defined(USE_TINYUSB)
+#define Serial SerialTinyUSB
 #endif
 
 #endif // else of ESP32

examples/CDC/cdc_multi/.funhouse.test.skip

@@ -76,7 +76,9 @@ USBWakeUp_IRQHandler(void) {
 
 void yield(void) {
   tud_task();
-  // flush cdc
+  if (tud_cdc_connected()) {
+    tud_cdc_write_flush();
+  }
 }
 }