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/pwmcmds.c

@@ -59,7 +59,7 @@ command_schedule_pwm_out(uint32_t *args)
 DECL_COMMAND(command_schedule_pwm_out,
              "schedule_pwm_out oid=%c clock=%u value=%hu");
 
-static void
+void
 pwm_shutdown(void)
 {
     uint8_t i;