Cleanup to DECLARE_string in header file, and make sure dependency is clearer.

Change-Id: I103575ef6d56a3f50da0c60ca44aacb3f02194e9
diff --git a/aos/network/message_bridge_test.cc b/aos/network/message_bridge_test.cc
index 96003f7..8d52292 100644
--- a/aos/network/message_bridge_test.cc
+++ b/aos/network/message_bridge_test.cc
@@ -7,9 +7,7 @@
 #include "aos/events/pong_generated.h"
 #include "aos/network/message_bridge_client_lib.h"
 #include "aos/network/message_bridge_server_lib.h"
-
-DECLARE_string(override_hostname);
-DECLARE_string(application_name);
+#include "aos/network/team_number.h"
 
 namespace aos {
 namespace message_bridge {
diff --git a/aos/network/team_number.cc b/aos/network/team_number.cc
index 8d4e69e..f690943 100644
--- a/aos/network/team_number.cc
+++ b/aos/network/team_number.cc
@@ -5,11 +5,11 @@
 #include <unistd.h>
 #include <stdlib.h>
 
-#include "glog/logging.h"
-
 #include "aos/util/string_to_num.h"
 
-DECLARE_string(override_hostname);
+DEFINE_string(override_hostname, "",
+    "If set, this forces the hostname of this node to be the provided "
+    "hostname.");
 
 namespace aos {
 namespace network {
diff --git a/aos/network/team_number.h b/aos/network/team_number.h
index 91a61f9..c2954b6 100644
--- a/aos/network/team_number.h
+++ b/aos/network/team_number.h
@@ -6,6 +6,10 @@
 #include <optional>
 #include <string>
 
+#include "glog/logging.h"
+
+DECLARE_string(override_hostname);
+
 namespace aos {
 namespace network {