MdeModulePkg DxeCore: Enhance MemoryAttributesTable installation

Current MemoryAttributesTable will be installed on ReadyToBoot event
at TPL_NOTIFY level, it maybe incorrect when PcdHiiOsRuntimeSupport
= TRUE as HiiDatabaseDxe will have runtime memory allocation for HII
OS runtime support on and after ReadyToBoot. The issue was exposed at
http://article.gmane.org/gmane.comp.bios.edk2.devel/10125.

To make sure the correctness of MemoryAttributesTable, this patch is
to enhance MemoryAttributesTable installation to install
MemoryAttributesTable on ReadyToBoot event at TPL_CALLBACK - 1 level
to make sure it is at the last of ReadyToBoot event, and also hook
runtime memory allocation after ReadyToBoot.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

MdeModulePkg/Core/Dxe/DxeMain/DxeMain.c

@@ -207,6 +207,7 @@ EFI_SYSTEM_TABLE      *gDxeCoreST = NULL;
 EFI_RUNTIME_SERVICES  *gDxeCoreRT = &mEfiRuntimeServicesTableTemplate;
 EFI_HANDLE            gDxeCoreImageHandle = NULL;
 
+BOOLEAN               gMemoryMapTerminated = FALSE;
 
 //
 // EFI Decompress Protocol
@@ -751,6 +752,8 @@ CoreExitBootServices (
     return Status;
   }
 
+  gMemoryMapTerminated = TRUE;
+
   //
   // Notify other drivers that we are exiting boot services.
   //