[sam] properly turn off usb code when USBCON is not defined

hardware/arduino/sam/cores/arduino/USB/USBCore.cpp

@@ -1,3 +1,5 @@
+#ifdef USBCON
+
 /* Copyright (c) 2010, Peter Barrett
 **
 ** Permission to use, copy, modify, and/or distribute this software for
@@ -587,3 +589,5 @@ bool USB_::configured()
 void USB_::poll()
 {
 }
+
+#endif // USBCON

hardware/arduino/sam/cores/arduino/validation_usb_device/test_usb_device.cpp

@@ -16,6 +16,8 @@
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
 
+#if defined(USBCON)
+
 #include "variant.h"
 #include <stdio.h>
 
@@ -36,3 +38,5 @@ void loop() {
 	}
 	delay(10);
 }
+
+#endif