Add option to specify a kernel subkey signing key to firmware signing utility.

utility/include/firmware_utility.h

Index: utility/include/firmware_utility.h
diff --git a/utility/include/firmware_utility.h b/utility/include/firmware_utility.h
index 663536a25cca48465e2373d858fe52d9d7ea1b00..27c154fa7d4ebdb72ab3e98906c8eb316715d5db 100644
--- a/utility/include/firmware_utility.h
+++ b/utility/include/firmware_utility.h
@@ -59,9 +59,11 @@ class FirmwareUtility {
   std::string subkey_in_file_;  // Existing key signature header.
   std::string in_file_;
   std::string out_file_;
+  std::string kernel_subkey_sign_pub_file_;
   int firmware_key_version_;
   int firmware_sign_algorithm_;
   int firmware_version_;
+  int kernel_subkey_sign_algorithm_;
   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?