Return EFI_UNSUPPORTED if READY_TO_BOOT function is invoked at SMM runtime.

Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Dong Guo <guo.dong@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13535 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.c

@@ -513,6 +513,10 @@ SmmVariableHandler (
       break;
 
     case SMM_VARIABLE_FUNCTION_READY_TO_BOOT:
+      if (AtRuntime()) {
+        Status = EFI_UNSUPPORTED;
+        break;
+      }
       ReclaimForOS ();
       Status = EFI_SUCCESS;
       break;