AP_Param: added an initialised() method

this will be used by the compass code

libraries/AP_Common/AP_Param.cpp

@@ -206,6 +206,12 @@ bool AP_Param::setup(const AP_Param::Info *info, uint8_t num_vars, uint16_t eepr
     return true;
 }
 
+// check if AP_Param has been initialised
+bool AP_Param::initialised(void)
+{
+    return _var_info != NULL;
+}
+
 // find the info structure given a header and a group_info table
 // return the Info structure and a pointer to the variables storage
 const struct AP_Param::Info *AP_Param::find_by_header_group(struct Param_header phdr, void **ptr,

libraries/AP_Common/AP_Param.h

@@ -80,6 +80,9 @@ public:
     // wrong version is found
     static bool setup(const struct Info *info, uint8_t num_vars, uint16_t eeprom_size);
 
+    // return true if AP_Param has been initialised via setup()
+    static bool initialised(void);
+
     /// Copy the variable's name, prefixed by any containing group name, to a buffer.
     ///
     /// If the variable has no name, the buffer will contain an empty string.