Landing slope move to standalone library

- this is shared by both the FW position controller and navigator's
   missiong feasibility checker

src/modules/navigator/CMakeLists.txt

@@ -56,4 +56,5 @@ px4_add_module(
 	DEPENDS
 		git_ecl
 		ecl_geo
+		landing_slope
 	)

src/modules/navigator/mission_feasibility_checker.cpp

@@ -45,9 +45,9 @@
 #include "navigator.h"
 
 #include <drivers/drv_pwm_output.h>
-#include <fw_pos_control_l1/Landingslope.hpp>
 #include <lib/ecl/geo/geo.h>
 #include <lib/mathlib/mathlib.h>
+#include <lib/landing_slope/Landingslope.hpp>
 #include <systemlib/mavlink_log.h>
 #include <uORB/Subscription.hpp>
 #include <uORB/topics/position_controller_landing_status.h>