Merge pull request #56 from duff2013/master

Fix for IDE Menu option "NO USB" @ F_CPU <= 16MHz LC/T3.x

teensy3/usb_inst.cpp

@@ -77,6 +77,8 @@ usb_serial_class Serial;
 
 #if defined(USB_SERIAL) || defined(USB_SERIAL_HID)
 usb_serial_class Serial;
+#elif (USB_DISABLED)
+usb_serial_class Serial;
 #else
 usb_seremu_class Serial;
 #endif