py: Rename MP_OBJ_NOT_SUPPORTED to MP_OBJ_NULL.

See issue #608 for justification.

py/objnone.c

@@ -44,7 +44,7 @@ STATIC void none_print(void (*print)(void *env, const char *fmt, ...), void *env
 STATIC mp_obj_t none_unary_op(int op, mp_obj_t o_in) {
     switch (op) {
         case MP_UNARY_OP_BOOL: return mp_const_false;
-        default: return MP_OBJ_NOT_SUPPORTED;
+        default: return MP_OBJ_NULL; // op not supported
     }
 }