build: Use compile_time_request system for init, tasks, and shutdown

Avoid using linker magic to define the init, task, and shutdown
functions.  Instead, use the compile_time_request system.  This
simplifies the build and produces more efficient code.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/stepper.c

@@ -284,7 +284,7 @@ stepper_stop(struct stepper *s)
     }
 }
 
-static void
+void
 stepper_shutdown(void)
 {
     uint8_t i;