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.

msg/EstimatorStatusFlags.msg

@@ -45,6 +45,8 @@ bool cs_mag_heading_consistent  # 37 - true when the heading obtained from mag d
 bool cs_aux_gpos                # 38 - true if auxiliary global position measurement fusion is intended
 bool cs_rng_terrain             # 39 - true if we are fusing range finder data for terrain
 bool cs_opt_flow_terrain        # 40 - true if we are fusing flow data for terrain
+bool cs_valid_fake_pos          # 41 - true if a valid constant position is being fused
+bool cs_constant_pos            # 42 - true if the vehicle is at a constant position
 
 # fault status
 uint32 fault_status_changes   # number of filter fault status (fs) changes