AP_NavEKF_Source: optimise configured_in_storage

This small CPU optimisation takes advantage of the fact that once configured in storage is true it will always remain true

libraries/AP_NavEKF/AP_NavEKF_Source.cpp

@@ -293,10 +293,16 @@ bool AP_NavEKF_Source::usingGPS() const
 }
 
 // true if some parameters have been configured (used during parameter conversion)
-bool AP_NavEKF_Source::configured_in_storage() const
+bool AP_NavEKF_Source::configured_in_storage()
 {
+    if (config_in_storage) {
+        return true;
+    }
+
     // first source parameter is used to determine if configured or not
-    return _source_set[0].posxy.configured_in_storage();
+    config_in_storage = _source_set[0].posxy.configured_in_storage();
+
+    return config_in_storage;
 }
 
 // mark parameters as configured in storage (used to ensure parameter conversion is only done once)

libraries/AP_NavEKF/AP_NavEKF_Source.h

@@ -83,7 +83,7 @@ public:
     bool usingGPS() const;
 
     // true if source parameters have been configured (used for parameter conversion)
-    bool configured_in_storage() const;
+    bool configured_in_storage();
 
     // mark parameters as configured in storage (used to ensure parameter conversion is only done once)
     void mark_configured_in_storage();
@@ -115,4 +115,5 @@ private:
         SourceYaw yaw;      // current yaw source
     } _active_source_set;
     bool initialised;       // true once init has been run
+    bool config_in_storage; // true once configured in storage has returned true
 };