run usb background when checking for Serial.available() if needed

Author: hathach

cores/arduino/Adafruit_TinyUSB_Core/Adafruit_USBD_CDC.cpp

@@ -67,15 +67,21 @@ Adafruit_USBD_CDC::operator bool()
   bool ret = tud_cdc_connected();
 
   // Add an yield to run usb background in case sketch block wait as follows
-  // while(!Serial) {}
+  // while( !Serial ) {}
   if ( !ret ) yield();
 
   return ret;
 }
 
 int Adafruit_USBD_CDC::available(void)
 {
-  return tud_cdc_available();
+  uint32_t count = tud_cdc_available();
+
+  // Add an yield to run usb background in case sketch block wait as follows
+  // while( !Serial.available() ) {}
+  if (!count) yield();
+
+  return count;
 }
 
 int Adafruit_USBD_CDC::peek(void)