Vboot Reference: Make length types explicitly sized.

Review URL: http://codereview.chromium.org/744002
diff --git a/utils/kernel_utility.cc b/utils/kernel_utility.cc
index 4e12c6d..8c4ae90 100644
--- a/utils/kernel_utility.cc
+++ b/utils/kernel_utility.cc
@@ -200,7 +200,7 @@
 }
 
 bool KernelUtility::GenerateSignedImage(void) {
-  uint32_t kernel_key_pub_len;
+  uint64_t kernel_key_pub_len;
   uint8_t* header_checksum;
   DigestContext ctx;
   image_ = KernelImageNew();