Add support for USB_DISABLED

teensy3/usb_dev.h

 #define USB_DESC_LIST_DEFINE
 #include "usb_desc.h"
 
-#if F_CPU >= 20000000
+#if F_CPU >= 20000000 && !defined(USB_DISABLED)
 
 // This header is NOT meant to be included when compiling
 // user sketches in Arduino.  The low-level functions

teensy3/usb_inst.cpp

 #if F_CPU >= 20000000
 
 #ifdef CDC_DATA_INTERFACE
+#ifdef CDC_STATUS_INTERFACE
 usb_serial_class Serial;
 #endif
+#endif
 
 #ifdef MIDI_INTERFACE
 usb_midi_class usbMIDI;
 uint8_t usb_joystick_class::manual_mode = 0;
 #endif
 
+#ifdef USB_DISABLED
+usb_serial_class Serial;
+#endif
+
 
 #else // F_CPU < 20 MHz
 

teensy3/usb_serial.h

 
 #include "usb_desc.h"
 
-#if defined(CDC_STATUS_INTERFACE) && defined(CDC_DATA_INTERFACE)
+#if (defined(CDC_STATUS_INTERFACE) && defined(CDC_DATA_INTERFACE)) || defined(USB_DISABLED)
 
 #include <inttypes.h>
 
-#if F_CPU >= 20000000
+#if F_CPU >= 20000000 && !defined(USB_DISABLED)
 
 // C language implementation
 #ifdef __cplusplus