Add drivetrain to y2017

Change-Id: I7494fd6c4b242ceb4aba26c9b5ddee064f29f382
diff --git a/y2017/constants.h b/y2017/constants.h
index be29a2a..3ca12c9 100644
--- a/y2017/constants.h
+++ b/y2017/constants.h
@@ -25,6 +25,8 @@
 
   // ///// Dynamic constants. /////
   double drivetrain_max_speed;
+
+  double down_error;
 };
 
 // Creates (once) a Values instance for ::aos::network::GetTeamNumber() and