AP_AHRS: make setting of home boolean in preparation for sanity checks

libraries/AP_AHRS/AP_AHRS.h

@@ -462,7 +462,7 @@ public:
     // set the home location in 10e7 degrees. This should be called
     // when the vehicle is at this position. It is assumed that the
     // current barometer and GPS altitudes correspond to this altitude
-    virtual void set_home(const Location &loc) = 0;
+    virtual bool set_home(const Location &loc) WARN_IF_UNUSED = 0;
 
     // set the EKF's origin location in 10e7 degrees.  This should only
     // be called when the EKF has no absolute position reference (i.e. GPS)

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -1031,7 +1031,7 @@ bool AP_AHRS_DCM::airspeed_estimate(float *airspeed_ret) const
     return ret;
 }
 
-void AP_AHRS_DCM::set_home(const Location &loc)
+bool AP_AHRS_DCM::set_home(const Location &loc)
 {
     _home = loc;
     _home_is_set = true;
@@ -1042,6 +1042,8 @@ void AP_AHRS_DCM::set_home(const Location &loc)
     // send new home and ekf origin to GCS
     gcs().send_home();
     gcs().send_ekf_origin();
+
+    return true;
 }
 
 //  a relative ground position to home in meters, Down

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -96,7 +96,7 @@ public:
 
     bool            use_compass() override;
 
-    void set_home(const Location &loc) override;
+    bool set_home(const Location &loc) override WARN_IF_UNUSED;
     void estimate_wind(void);
 
     // is the AHRS subsystem healthy?