Add maximum length constant for HWID, so it can be used by other utilities/firmware.

firmware/include/gbb_header.h

 /* 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.
  *
  * Data structure of Google Binary Block (GBB)
  */
 
 #ifndef VBOOT_REFERENCE_GBB_HEADER_H_
 #define VBOOT_REFERENCE_GBB_HEADER_H_
 
 #include "sysincludes.h"
 
 #define GBB_HEADER_SIZE    (0x80)
 
 #define GBB_SIGNATURE      "$GBB"
 #define GBB_SIGNATURE_SIZE (4)
 
 #define GBB_MAJOR_VER      (0x01)
 #define GBB_MINOR_VER      (0x00)
 
+/* Maximum length of a HWID in bytes, counting terminating null. */
+#define GBB_HWID_MAX_SIZE  256
+
 #ifdef __cplusplus
 extern "C" {
 #endif  // __cplusplus
 
 typedef struct GoogleBinaryBlockHeader {
   uint8_t  signature[GBB_SIGNATURE_SIZE]; // GBB_SIGNATURE "$GBB"
   uint16_t major_version;   // see GBB_MAJOR_VER
   uint16_t minor_version;   // see GBB_MINOR_VER
   uint32_t header_size;     // size of GBB header in bytes
   uint32_t reserved;
 
   uint32_t hwid_offset;     // HWID offset from header
   uint32_t hwid_size;       // HWID size in bytes
   uint32_t rootkey_offset;  // Root Key offset from header
   uint32_t rootkey_size;    // Root Key size in bytes
   uint32_t bmpfv_offset;    // BMP FV offset from header
   uint32_t bmpfv_size;      // BMP FV size in bytes
   uint32_t recovery_key_offset;  // Recovery Key offset from header
   uint32_t recovery_key_size;    // Recovery Key size in bytes
 
   uint8_t  pad[80];         // to match GBB_HEADER_SIZE
 } GoogleBinaryBlockHeader;
 
 #ifdef __cplusplus
 }
 #endif  // __cplusplus
 
 #endif  /* VBOOT_REFERENCE_GBB_HEADER_H_ */