Add a kernel subkey signing algorithm key and algorithm fields to firmware preamble.

src/platform/vboot_reference/vboot_firmware/include/firmware_image_fw.h

Index: src/platform/vboot_reference/vboot_firmware/include/firmware_image_fw.h
diff --git a/src/platform/vboot_reference/vboot_firmware/include/firmware_image_fw.h b/src/platform/vboot_reference/vboot_firmware/include/firmware_image_fw.h
index 8cba176c4bd127555041b3ee09269bd2383fe89b..fb90f0271b8d8f6e67dc80ab657eca52f9ed4012 100644
--- a/src/platform/vboot_reference/vboot_firmware/include/firmware_image_fw.h
+++ b/src/platform/vboot_reference/vboot_firmware/include/firmware_image_fw.h
@@ -36,6 +36,10 @@ typedef struct FirmwareImage {
   /* Firmware Preamble. */
   uint16_t firmware_version;  /* Firmware Version# for preventing rollbacks.*/
   uint64_t firmware_len;  /* Length of the rest of the R/W firmware data. */
+  uint16_t kernel_subkey_sign_algorithm;  /* Signature algorithm used for
+                                           * signing the kernel subkey. */
+  uint8_t* kernel_subkey_sign_key;  /* Pre-processed public half of the kernel
+                                     * subkey signing key. */
   uint8_t preamble[FIRMWARE_PREAMBLE_SIZE];  /* Remaining preamble data.*/
 
   uint8_t* preamble_signature;  /* Signature over the preamble. */
@@ -65,6 +69,10 @@ typedef struct FirmwareImage {
 
 extern char* kVerifyFirmwareErrors[VERIFY_FIRMWARE_MAX];
 
+/* Returns the length of the verified boot firmware preamble based on
+ * kernel subkey signing algorithm [algorithm]. */
+uint64_t GetFirmwarePreambleLen(int algorithm);
+
 /* Checks for the sanity of the firmware header pointed by [header_blob].
  *
  * On success, put signature algorithm in [algorithm], header length