Add --subkey_in and --subkey_out options to firmware signing utility.

src/platform/vboot_reference/vkernel/include/kernel_image.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.
  *
  * API definitions for a generating and manipulating verified boot kernel images.
  * (Userland portion.)
  */
 
 #ifndef VBOOT_REFERENCE_KERNEL_IMAGE_H_
 #define VBOOT_REFERENCE_KERNEL_IMAGE_H_
@@ skipping 38 matching lines @@
 
 /* Get a verified kernel binary blob from an [image] and fill
  * its length into blob_len.
  *
  * Caller owns the returned pointer and must Free() it.
  */
 uint8_t* GetKernelBlob(const KernelImage* image, uint64_t* blob_len);
 
 
 /* Write kernel data from [image] to a file named [input_file].
- * If [is_only_vblock] is non-zero, only the verification block is output.
+ *
+ * If [is_only_vblock] is non-zero, only the verification block (excluding the
+ * actual kernel data) is output.
+ * If [is_subkey_out] is non-zero, only the kernel key verification (subkey)
+ * header is output.
  *
  * Return 1 on success, 0 on error.
  */
 int WriteKernelImage(const char* input_file,
                      const KernelImage* image,
                      int is_only_vblock,
                      int is_subkey_out);
 
 /* Create a kernel_data blob from its components and fill
  * its length into blob_len, plus some information about the bootloader.
@@ skipping 37 matching lines @@
  * using the private signing key in file [kernel_sigining_key_file].
  *
  * Return 1 on success, 0 on failure.
  */
 int AddKernelSignature(KernelImage* image,
                        const char* kernel_sigining_key_file);
 
 void PrintKernelEntry(kernel_entry* entry);
 
 #endif  /* VBOOT_REFERENCE_KERNEL_IMAGE_H_ */