Simplify ping and pong

The simple demo apps grew to the point where they were no longer simple
and easy to understand since I was using them to test out timing reports
and accidentally committed the changes.  Restore the simplicity.

Change-Id: I453fa0351247c4549098660cd21191659269f515
diff --git a/aos/events/ping.cc b/aos/events/ping.cc
index 5a79635..f829b41 100644
--- a/aos/events/ping.cc
+++ b/aos/events/ping.cc
@@ -6,11 +6,13 @@
 #include "gflags/gflags.h"
 #include "glog/logging.h"
 
+DEFINE_string(config, "aos/events/pingpong_config.json", "Path to the config.");
+
 int main(int argc, char **argv) {
   aos::InitGoogle(&argc, &argv);
 
   aos::FlatbufferDetachedBuffer<aos::Configuration> config =
-      aos::configuration::ReadConfig("aos/events/pingpong_config.json");
+      aos::configuration::ReadConfig(FLAGS_config);
 
   aos::ShmEventLoop event_loop(&config.message());