ekf2-fake pos: add valid fake position fusion

This is similar to fake pos but is only used when the ekf has an
external information telling it that the vehicle is not changing
position. This information can then be used to keep a valid local
position even when the vehicle isn't exactly at rest.

src/modules/ekf2/EKF/common.h

@@ -261,6 +261,7 @@ struct systemFlagUpdate {
 	bool in_air{true};
 	bool is_fixed_wing{false};
 	bool gnd_effect{false};
+	bool constant_pos{false};
 };
 
 struct parameters {
@@ -614,6 +615,8 @@ uint64_t mag_heading_consistent  :
 		uint64_t aux_gpos                : 1; ///< 38 - true if auxiliary global position measurement fusion is intended
 		uint64_t rng_terrain             : 1; ///< 39 - true if we are fusing range finder data for terrain
 		uint64_t opt_flow_terrain        : 1; ///< 40 - true if we are fusing flow data for terrain
+		uint64_t valid_fake_pos          : 1; ///< 41 - true if a valid constant position is being fused
+		uint64_t constant_pos            : 1; ///< 42 - true if the vehicle is at a constant position
 
 	} flags;
 	uint64_t value;