AP_Param: expose the stable key for use by AP_OSD

add support for finding parameters by name and returning the token

libraries/AP_Param/AP_Param.cpp

@@ -909,6 +909,26 @@ AP_Param::find_by_index(uint16_t idx, enum ap_var_type *ptype, ParamToken *token
     return ap;    
 }
 
+// by-name equivalent of find_by_index()
+AP_Param* AP_Param::find_by_name(const char* name, enum ap_var_type *ptype, ParamToken *token)
+{
+    AP_Param *ap;
+    uint16_t count = 0;
+    for (ap = AP_Param::first(token, ptype);
+         ap && *ptype != AP_PARAM_GROUP && *ptype != AP_PARAM_NONE;
+         ap = AP_Param::next_scalar(token, ptype)) {
+        int32_t ret = strncasecmp(name, _var_info[token->key].name, AP_MAX_NAME_SIZE);
+        if (ret >= 0) {
+            char buf[AP_MAX_NAME_SIZE];
+            ap->copy_name_token(*token, buf, AP_MAX_NAME_SIZE);
+            if (strncasecmp(name, buf, AP_MAX_NAME_SIZE) == 0) {
+                break;
+            }
+        }
+        count++;
+    }
+    return ap;
+}
 
 /*
   Find a variable by pointer, returning key. This is used for loading pointer variables

libraries/AP_Param/AP_Param.h

@@ -314,6 +314,9 @@ public:
     ///
     static AP_Param * find_by_index(uint16_t idx, enum ap_var_type *ptype, ParamToken *token);
 
+    // by-name equivalent of find_by_index()
+    static AP_Param* find_by_name(const char* name, enum ap_var_type *ptype, ParamToken *token);
+
     /// Find a variable by pointer
     ///
     ///
@@ -475,6 +478,9 @@ public:
     // return true if the parameter is read-only
     bool is_read_only(void) const;
 
+    // return the persistent top level key for the ParamToken key
+    static uint16_t get_persistent_key(uint16_t key) { return _var_info[key].key; }
+    
     // count of parameters in tree
     static uint16_t count_parameters(void);