Merge pull request #829 from TanekLiang/iar_unused

Add RT_USED definition for IAR

include/rtdef.h

@@ -110,7 +110,7 @@ typedef rt_base_t                       rt_off_t;       /**< Type for offset */
     #include <stdarg.h>
     #define SECTION(x)                  @ x
     #define RT_UNUSED
-    #define RT_USED
+    #define RT_USED                     __root
     #define PRAGMA(x)                   _Pragma(#x)
     #define ALIGN(n)                    PRAGMA(data_alignment=n)
     #define WEAK                        __weak