Rearrange vboot_reference directories to isolate external components.

src/platform/vboot_reference/vboot_firmware/stub/boot_device_stub.c

 /* Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  *
  * Stub implementations of boot device functions.
  */
 
 #include "boot_device.h"
 
 int BootDeviceReadLBA(uint64_t lba_start, uint64_t lba_count, void *buffer) {
   return 1;
 }
 
 int BootDeviceWriteLBA(uint64_t lba_start, uint64_t lba_count,
                        const void *buffer) {
   return 1;
 }