Use explicit flatbuffer IDs in y2017 and newer.

Non-explicit ids are risky.  We've seen backwards incompatible
changes...

Change-Id: Id6ceebe031ac80430191f367635d0e951c3d2cbc
diff --git a/y2020/setpoint.fbs b/y2020/setpoint.fbs
index 5f1af72..74164e5 100644
--- a/y2020/setpoint.fbs
+++ b/y2020/setpoint.fbs
@@ -1,11 +1,11 @@
 namespace y2020.joysticks;
 
 table Setpoint {
-  accelerator:double;
+  accelerator:double (id: 0);
 
-  finisher:double;
+  finisher:double (id: 1);
 
-  hood:double;
+  hood:double (id: 2);
 }
 
 root_type Setpoint;