allows use of "myboard.build.vid" in boards.txt

allows people to create board that depends on sam core to use their own Vendor ID instead of Arduino's

Author: Phil Manofsky

hardware/arduino/sam/cores/arduino/Arduino.h

@@ -209,10 +209,14 @@ extern const PinDescription g_APinDescription[] ;
 #include "watchdog.h"
 
 // USB Device
+#ifndef USB_VID
 #define USB_VID            0x2341 // arduino LLC vid
+#endif
+
 #define USB_PID_LEONARDO   0x0034
 #define USB_PID_MICRO      0x0035
 #define USB_PID_DUE        0x003E
+
 #include "USB/USBDesc.h"
 #include "USB/USBCore.h"
 #include "USB/USBAPI.h"