Add a blank screen value into screen index.

firmware/include/bmpblk_header.h

 /* Copyright (c) 2010-2011 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.
  *
  * Data structure definitions for firmware screen block (BMPBLOCK).
  *
  * The BmpBlock structure looks like:
  *  +-----------------------------------------+
  *  |             BmpBlock Header             |
  *  +-----------------------------------------+
@@ skipping 67 matching lines @@
   } images[MAX_IMAGE_IN_LAYOUT];  /* Images contained in the screen. Will be
                                    * rendered from 0 to (number_of_images-1). */
 } __attribute__((packed)) ScreenLayout;
 
 /* Constants for screen index */
 typedef enum ScreenIndex {
   SCREEN_DEVELOPER_MODE = 0,
   SCREEN_RECOVERY_MODE,
   SCREEN_RECOVERY_NO_OS,
   SCREEN_RECOVERY_MISSING_OS,
-  MAX_SCREEN_INDEX,
+  MAX_VALID_SCREEN_INDEX,
+  SCREEN_BLANK = ~0UL,
 } ScreenIndex;
 
 /* Image info, describing the information of the image block */
 typedef struct ImageInfo {
   uint32_t tag;              /* Tag it as a special image, like HWID */
   uint32_t width;            /* Width of the image */
   uint32_t height;           /* Height of the image */
   uint32_t format;           /* File format of the image */
   uint32_t compression;      /* Compression method for the image file */
   uint32_t original_size;    /* Size of the original uncompressed image */
@@ skipping 18 matching lines @@
 typedef enum Compression {
   COMPRESS_NONE = 0,
   COMPRESS_EFIv1,           /* The x86 BIOS only supports this */
   COMPRESS_LZMA1,           /* The ARM BIOS supports LZMA1 */
   MAX_COMPRESS,
 } Compression;
 
 __pragma(pack(pop)) /* Support packing for MSVC. */
 
 #endif  /* VBOOT_REFERENCE_BMPBLK_HEADER_H_ */