Flatbuffer housekeeping in EventLoop.

We should probably use configuration() to be more flexible instead of
configuration_, and now is a good time to add a
Flatbuffer<Configuration> constructor to ShmEventLoop to prevent a bunch
of copy-paste code.

Change-Id: Ic32e131fdb9a68bc9305dd5a48eaf3101dd796b3
diff --git a/aos/events/shm_event_loop.h b/aos/events/shm_event_loop.h
index a204a30..148a206 100644
--- a/aos/events/shm_event_loop.h
+++ b/aos/events/shm_event_loop.h
@@ -34,6 +34,8 @@
 // destructors are called back in one thread again)
 class ShmEventLoop : public EventLoop {
  public:
+  ShmEventLoop(const Flatbuffer<Configuration> &configuration)
+      : ShmEventLoop(&configuration.message()) {}
   ShmEventLoop(const Configuration *configuration);
   ~ShmEventLoop() override;