Convert aos over to flatbuffers

Everything builds, and all the tests pass.  I suspect that some entries
are missing from the config files, but those will be found pretty
quickly on startup.

There is no logging or live introspection of queue messages.

Change-Id: I496ee01ed68f202c7851bed7e8786cee30df29f5
diff --git a/frc971/control_loops/drivetrain/drivetrain_output.fbs b/frc971/control_loops/drivetrain/drivetrain_output.fbs
new file mode 100644
index 0000000..da8c889
--- /dev/null
+++ b/frc971/control_loops/drivetrain/drivetrain_output.fbs
@@ -0,0 +1,13 @@
+namespace frc971.control_loops.drivetrain;
+
+table Output {
+  // Voltage to send to motor(s) on either side of the drivetrain.
+  left_voltage:double;
+  right_voltage:double;
+
+  // Whether to set each shifter piston to high gear.
+  left_high:bool;
+  right_high:bool;
+}
+
+root_type Output;