NetworkPkg: Add IPv6 DUID configuration support using platform PCD.

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