Add ids to flatbuffer fields in y2012, y2016, frc971, and aos

Change-Id: I9ed9006ce6224e2df0459df47771786b928164a1
diff --git a/aos/controls/control_loops.fbs b/aos/controls/control_loops.fbs
index 8456bfa..be5b7db 100644
--- a/aos/controls/control_loops.fbs
+++ b/aos/controls/control_loops.fbs
@@ -3,19 +3,19 @@
 // control loop.
 
 table Goal {
-  goal:double;
+  goal:double (id: 0);
 }
 
 table Position {
-  position:double;
+  position:double (id: 0);
 }
 
 table Output {
-  voltage:double;
+  voltage:double (id: 0);
 }
 
 table Status {
-  done:bool;
+  done:bool (id: 0);
 }
 
 root_type Goal;