Port PlatformDriOverrideDxe into R9.

1. Add one OverrideVariableGuid in MdeModulePkg
2. Add one library PlatDriOverLib used by PlatformDriOverrideDxe.
3. Add PlatformDriOverrideDxe in MdeModulePkg

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@4557 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Include/Guid/OverrideVariable.h

@@ -0,0 +1,34 @@
+/*++
+
+Copyright (c) 2008, Intel Corporation
+All rights reserved. This program and the accompanying materials
+are licensed and made available under the terms and conditions of the BSD License
+which accompanies this distribution.  The full text of the license may be found at
+http://opensource.org/licenses/bsd-license.php
+
+THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+
+Module Name:
+
+  OverrideVariable.h
+
+Abstract:
+
+  Platform Override Variable Guid definitions
+
+--*/
+
+#ifndef __EFI_OVERRIDE_VARIABLE_GUID_H__
+#define __EFI_OVERRIDE_VARIABLE_GUID_H__
+
+//
+// This guid is used for a platform driver override variable
+//
+#define EFI_OVERRIDE_VARIABLE_GUID  \
+  { 0x8e3d4ad5, 0xf762, 0x438a, { 0xa1, 0xc1, 0x5b, 0x9f, 0xe6, 0x8c, 0x6b, 0x15 }}
+
+extern EFI_GUID gEfiOverrideVariableGuid;
+
+
+#endif // #ifndef __EFI_OVERRIDE_VARIABLE_GUID_H__