Use enums for drivetrain KF states

This makes things a lot easier to follow.

Change-Id: Ib3d2b60fd8f77851cf842340447d92905b69cb75
diff --git a/frc971/control_loops/drivetrain/BUILD b/frc971/control_loops/drivetrain/BUILD
index d42b97c..cf6b19f 100644
--- a/frc971/control_loops/drivetrain/BUILD
+++ b/frc971/control_loops/drivetrain/BUILD
@@ -285,6 +285,7 @@
         ":drivetrain_config",
         ":drivetrain_goal_fbs",
         ":drivetrain_output_fbs",
+        ":drivetrain_states",
         ":drivetrain_status_fbs",
         ":gear",
         ":localizer",
@@ -315,6 +316,7 @@
     }),
     deps = [
         ":drivetrain_config",
+        ":drivetrain_states",
         ":gear",
         "//aos:math",
         "//aos/controls:polytope",
@@ -368,6 +370,11 @@
 )
 
 cc_library(
+    name = "drivetrain_states",
+    hdrs = ["drivetrain_states.h"],
+)
+
+cc_library(
     name = "drivetrain_lib",
     srcs = [
         "drivetrain.cc",
@@ -381,6 +388,7 @@
         ":drivetrain_goal_fbs",
         ":drivetrain_output_fbs",
         ":drivetrain_position_fbs",
+        ":drivetrain_states",
         ":drivetrain_status_fbs",
         ":gear",
         ":improved_down_estimator",