bmc: add /flash/update to firmare handler

The /flash/update blob id will only appear when it's an available option
(after a successful image verification).  Add a Session for tracking
access to /flash/update.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I9e76c5217d4a71346c959061fb51418e3aa39c81
diff --git a/util.hpp b/util.hpp
index f304b1c..fd3a818 100644
--- a/util.hpp
+++ b/util.hpp
@@ -5,6 +5,7 @@
 namespace ipmi_flash
 {
 
+extern const std::string updateBlobId;
 extern const std::string verifyBlobId;
 extern const std::string hashBlobId;
 extern const std::string activeImageBlobId;