1. Update PXE driver to support PXEv6 boot cross subnet.

1. Update PXE driver to support PXEv6 boot cross subnet.
2. Update IP6 driver to use previous configured prefix length if a pre-exist IP6 address with unspecified prefix length.
3. Add NULL check for Dhcp protocol pointer before it decline the address in Ip6ConfigSetStatefulAddrCallback() function.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu, Siyuan <siyuan.fu@intel.com>
Reviewed-By: Ye, Ting (ting.ye@intel.com)
Reviewed-By: Wu, Jiaxin <jiaxin.wu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16131 6f19259b-4bc3-4df7-8a09-765794883524