Add a --subkey switch to kernel signing utility.

src/platform/vboot_reference/utility/include/kernel_utility.h

Index: src/platform/vboot_reference/utility/include/kernel_utility.h
diff --git a/src/platform/vboot_reference/utility/include/kernel_utility.h b/src/platform/vboot_reference/utility/include/kernel_utility.h
index 1d03685aa51f0c7e0dd3b997247a2b901730a531..66802cf3c66c959c4996ea450b0e170dcbf90edc 100644
--- a/src/platform/vboot_reference/utility/include/kernel_utility.h
+++ b/src/platform/vboot_reference/utility/include/kernel_utility.h
@@ -77,7 +77,8 @@ class KernelUtility {
   bool is_generate_;  // Are we generating a new image?
   bool is_verify_;  // Are we just verifying an already signed image?
   bool is_describe_;  // Should we print out description of the image?
-  bool is_only_vblock_;  // Show we just output the verification block?
+  bool is_only_vblock_;  // Should we just output the verification block?
+  bool is_subkey_out_;  // Should we just output the subkey header?
 };
 
 }  // namespace vboot_reference