Add 2020 drivetrain replay binary

Also, add a small comment to the 2020 wpilib_interface.

Change-Id: Ie6e070566884f535db459dd15fb54c2069c02379
diff --git a/y2020/control_loops/drivetrain/BUILD b/y2020/control_loops/drivetrain/BUILD
index d03c1b9..9abb2a1 100644
--- a/y2020/control_loops/drivetrain/BUILD
+++ b/y2020/control_loops/drivetrain/BUILD
@@ -81,3 +81,20 @@
         "//frc971/control_loops/drivetrain:drivetrain_lib",
     ],
 )
+
+cc_binary(
+    name = "drivetrain_replay",
+    srcs = ["drivetrain_replay.cc"],
+    data = ["//y2020:config.json"],
+    deps = [
+        ":drivetrain_base",
+        "//aos:configuration",
+        "//aos:init",
+        "//aos/events:shm_event_loop",
+        "//aos/events:simulated_event_loop",
+        "//aos/events/logging:logger",
+        "//frc971/control_loops/drivetrain:drivetrain_lib",
+        "@com_github_gflags_gflags//:gflags",
+        "@com_github_google_glog//:glog",
+    ],
+)