doc: create additional group for utils

doc.txt

+/**
+ * @defgroup utils Utilities
+ * @brief Utilities and helper functionality
+ *
+ *        Additional scripts and configuration options to ease RIOT
+ *        development.
+ * @{
+ */
+
 /**
  * @def DEVELHELP
  * @brief   This global macro activates some behavior that helps you while
  *          developing but is otherwise optimized out.
  */
+#if DOXYGEN
+#   define DEVELHELP
+#endif
 
 /**
  * @def TEST_SUITES
  * @brief   This global macro activates functionality that is needed for
  *          automated testing but not needed otherwise.
  */
+#if DOXYGEN
+#   define TEST_SUITES
+#endif
+
+/**
+ * @}
+ */