Add --subkey_in option 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 66802cf3c66c959c4996ea450b0e170dcbf90edc..12ad9b83bc2590212018b764c2a4f7a3da080d2b 100644
--- a/src/platform/vboot_reference/utility/include/kernel_utility.h
+++ b/src/platform/vboot_reference/utility/include/kernel_utility.h
@@ -8,7 +8,7 @@
 #include <string>
 
 extern "C" {
-#include  "kernel_image.h"
+#include "kernel_image.h"
 }
 
 struct RSAPublicKey;
@@ -58,6 +58,7 @@ class KernelUtility {
   std::string firmware_key_pub_file_;
   std::string kernel_key_file_;  // Private key for signing the kernel.
   std::string kernel_key_pub_file_;
+  std::string subkey_in_file_;  // Existing key signature header.
   std::string config_file_;  // File containing kernel commandline parameters
   std::string bootloader_file_;  // Embedded bootloader code
   std::string vmlinuz_file_;  // Input vmlinuz to be embedded in signed blob.