Add support for using external signing application and .pem private key files to vbutil_keyblock.

host/include/host_keyblock.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.
  *
  * Host-side functions for verified boot.
  */
 
 #ifndef VBOOT_REFERENCE_HOST_KEYBLOCK_H_
 #define VBOOT_REFERENCE_HOST_KEYBLOCK_H_
 
 #include "host_key.h"
 #include "vboot_struct.h"
 
 
 /* Create a key block header containing [data_key] and [flags], signed
+ * by private key the file [signing_key_pem_file] and algorithm [algorithm]
+ * using the external signer program [external_signer] for all private key
+ * operations.
+ * Caller owns the returned pointer, and must free
+ * it with Free(). */
+VbKeyBlockHeader* KeyBlockCreate_external(const VbPublicKey* data_key,
+                                          const char* signing_key_pem_file,
+                                          uint64_t algorithm,
+                                          uint64_t flags,
+                                          const char* external_signer);
+
+/* Create a key block header containing [data_key] and [flags], signed
  * by [signing_key].  Caller owns the returned pointer, and must free
  * it with Free(). */
 VbKeyBlockHeader* KeyBlockCreate(const VbPublicKey* data_key,
                                  const VbPrivateKey* signing_key,
                                  uint64_t flags);
 
 
 /* Read a key block from a .keyblock file.  Caller owns the returned
  * pointer, and must free it with Free().
  *
  * Returns NULL if error. */
 VbKeyBlockHeader* KeyBlockRead(const char* filename);
 
 
 /* Write a key block to a file in .keyblock format. */
 int KeyBlockWrite(const char* filename, const VbKeyBlockHeader* key_block);
 
 #endif  /* VBOOT_REFERENCE_HOST_KEYBLOCK_H_ */