NetworkPkg: Add IPv6 DUID configuration support using platform PCD.

The code today always uses DUID-UUID unless there is an error in reading the UUID, 
which results in falling through to DUID-LLT. This change allows the platform to
 control the DUID method (DUID-UUID or DUID-LLT) using a platform dynamic PCD.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Samer El-Haj-Mahmoud <elhaj@hp.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17751 6f19259b-4bc3-4df7-8a09-765794883524

NetworkPkg/Dhcp6Dxe/Dhcp6Utility.c

@@ -1,6 +1,7 @@
 /** @file
   Dhcp6 support functions implementation.
 
+  (C) Copyright 2015 Hewlett-Packard Development Company, L.P.<BR>
   Copyright (c) 2009 - 2014, Intel Corporation. All rights reserved.<BR>
 
   This program and the accompanying materials
@@ -63,7 +64,7 @@ Dhcp6GenerateClientId (
   //
   // If System UUID is found from SMBIOS Table, use DUID-UUID type.
   //
-  if (!EFI_ERROR (NetLibGetSystemGuid (&Uuid))) {
+  if ((PcdGet8 (PcdDhcp6UidType) == Dhcp6DuidTypeUuid) && !EFI_ERROR (NetLibGetSystemGuid (&Uuid))) {
     //
     //
     //  The format of DUID-UUID: