HAL_Linux: added --defaults option to Linux HAL

useful for initial parameters

libraries/AP_HAL_Linux/HAL_Linux_Class.cpp

@@ -311,6 +311,7 @@ void HAL_Linux::run(int argc, char* const argv[], Callbacks* callbacks) const
         {"terrain-directory",   true,  0, 't'},
         {"storage-directory",   true,  0, 's'},
         {"module-directory",    true,  0, 'M'},
+        {"defaults",            true,  0, 'd'},
         {"help",                false,  0, 'h'},
         {0, false, 0, 0}
     };
@@ -364,6 +365,9 @@ void HAL_Linux::run(int argc, char* const argv[], Callbacks* callbacks) const
             module_path = gopt.optarg;
             break;
 #endif
+        case 'd':
+            utilInstance.set_custom_defaults_path(gopt.optarg);
+            break;
         case 'h':
             _usage();
             exit(0);

libraries/AP_HAL_Linux/Util.h

@@ -50,6 +50,14 @@ public:
     void set_custom_storage_directory(const char *_custom_storage_directory) {
         custom_storage_directory = _custom_storage_directory;
     }
+    void set_custom_defaults_path(const char *_custom_defaults) {
+        custom_defaults = _custom_defaults;
+    }
+
+    // get path to custom defaults file for AP_Param
+    const char* get_custom_defaults_file() const override final {
+        return custom_defaults;
+    }
 
     bool is_chardev_node(const char *path);
     void set_imu_temp(float current) override;
@@ -119,6 +127,7 @@ private:
     const char *custom_log_directory = nullptr;
     const char *custom_terrain_directory = nullptr;
     const char *custom_storage_directory = nullptr;
+    const char *custom_defaults = HAL_PARAM_DEFAULTS_PATH;
     static const char *_hw_names[UTIL_NUM_HARDWARES];
 
 #ifdef ENABLE_HEAP