firmware_handler: implement session stat w/ verification

Implement session stat to handle checking on the verification state as
well as properly reporting when there isn't an image handler.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I98079a0f38976e1aa9876c8a2232481231dbc6e1
diff --git a/firmware_handler.hpp b/firmware_handler.hpp
index 212e570..fe410a1 100644
--- a/firmware_handler.hpp
+++ b/firmware_handler.hpp
@@ -111,7 +111,7 @@
     };
 
     /** The return values for verification. */
-    enum VerifyCheckResponses : std::uint8_t
+    enum class VerifyCheckResponses : std::uint8_t
     {
         running = 0,
         success = 1,