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.cpp b/util.cpp
index 05780b8..be18d72 100644
--- a/util.cpp
+++ b/util.cpp
@@ -19,6 +19,7 @@
 namespace ipmi_flash
 {
 
+const std::string updateBlobId = "/flash/update";
 const std::string verifyBlobId = "/flash/verify";
 const std::string hashBlobId = "/flash/hash";
 const std::string activeImageBlobId = "/flash/active/image";