Convert drivetrain mode from a bool to a number

This lets us support more than 2 drivetrain controller types in
perparation for splines.

Change-Id: I943d6073e6c5facae7223cc6111551a57b039a04
diff --git a/frc971/control_loops/drivetrain/drivetrain.q b/frc971/control_loops/drivetrain/drivetrain.q
index ed659c3..7fb859d 100644
--- a/frc971/control_loops/drivetrain/drivetrain.q
+++ b/frc971/control_loops/drivetrain/drivetrain.q
@@ -55,8 +55,11 @@
     // True to activate quickturn.
     bool quickturn;
 
-    // True to have the closed-loop controller take over.
-    bool control_loop_driving;
+    // Type of controller in charge of the drivetrain.
+    //  0: polydrive
+    //  1: motion profiled position drive (statespace)
+    //  2: spline follower.
+    uint8_t controller_type;
 
     // Position goals for each drivetrain side (in meters) when the
     // closed-loop controller is active.