Removed extra abstracton layer in systemlib

The calls to task_spawn_cmd, kill_all, and systemreset  were wrappers
around the px4_{task_spawn_cmd|kill_all|systemreset} implementations.

Removed the wrappers and changed all calls to the px4_ equivalents.

NuttX specific code was moved into px4_tasks.h

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/position_estimator_inav/position_estimator_inav_main.c

@@ -153,7 +153,7 @@ int position_estimator_inav_main(int argc, char *argv[])
 			}
 
 		thread_should_exit = false;
-		position_estimator_inav_task = task_spawn_cmd("position_estimator_inav",
+		position_estimator_inav_task = px4_task_spawn_cmd("position_estimator_inav",
 					       SCHED_DEFAULT, SCHED_PRIORITY_MAX - 5, 5000,
 					       position_estimator_inav_thread_main,
 					       (argv) ? (char * const *) &argv[2] : (char * const *) NULL);