AP_Param: added set_enable() API

used to set value of an enable parameter, clearing count cache

libraries/AP_Param/AP_Param.cpp

@@ -1074,7 +1074,7 @@ void AP_Param::save_sync(bool force_save)
 
     if (phdr.type == AP_PARAM_INT8 && ginfo != nullptr && (ginfo->flags & AP_PARAM_FLAG_ENABLE)) {
         // clear cached parameter count
-        _parameter_count = 0;
+        invalidate_count();
     }
     
     char name[AP_MAX_NAME_SIZE+1];
@@ -1476,7 +1476,7 @@ void AP_Param::load_object_from_eeprom(const void *object_pointer, const struct
     uint16_t key;
 
     // reset cached param counter as we may be loading a dynamic var_info
-    _parameter_count = 0;
+    invalidate_count();
     
     if (!find_key_by_pointer(object_pointer, key)) {
         hal.console->printf("ERROR: Unable to find param pointer\n");
@@ -2340,6 +2340,14 @@ uint16_t AP_Param::count_parameters(void)
     return ret;
 }
 
+/*
+  invalidate parameter count cache
+ */
+void AP_Param::invalidate_count(void)
+{
+    _parameter_count = 0;
+}
+
 /*
   set a default value by name
  */

libraries/AP_Param/AP_Param.h

@@ -478,6 +478,9 @@ public:
     // count of parameters in tree
     static uint16_t count_parameters(void);
 
+    // invalidate parameter count
+    static void invalidate_count(void);
+
     static void set_hide_disabled_groups(bool value) { _hide_disabled_groups = value; }
 
     // set frame type flags. Used to unhide frame specific parameters
@@ -735,6 +738,14 @@ public:
         _value = v;
     }
 
+    // set a parameter that is an ENABLE param
+    void set_enable(const T &v) {
+        if (v != _value) {
+            invalidate_count();
+        }
+        _value = v;
+    }
+    
     /// Sets if the parameter is unconfigured
     ///
     void set_default(const T &v) {