SerialUSB shadowing: properly call begin() during init

Author: facchinm

cores/arduino/main.cpp

@@ -41,7 +41,7 @@ int main(void)
 
 #if defined(SERIAL_CDC)
   PluggableUSBD().begin();
-  SerialUSB.begin(115200);
+  _SerialUSB.begin(115200);
 #endif
 
 	setup();