BLE: Export byte_array_t size exploitable at compile time.

features/FEATURE_BLE/ble/BLETypes.h

@@ -263,6 +263,11 @@ private:
 
 template <size_t array_size>
 struct byte_array_t {
+    /**
+     * Size of the array; accessible at compile time.
+     */
+    static const size_t size_ = array_size;
+
     /**
      * Default to all zeroes
      */