NetworkPkg/UefiPxeBcDxe:Add two PCD to control PXE.

NetworkPkg/UefiPxeBcDxe:Add two PCD to control PXE.

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1695

Setup need provide an item for user to control IPV46 PXE boot.
Origin UefiPxeBcDxe driver doesn't have such interface.
This change added two PCD to control IPV4/6 PXE in PxeBcSupported().
Platform code should override this two PCD according to Setup value.
code change no side effect on current PXE function with default PCD.

Signed-off-by: Xue ShengfengX <shengfengx.xue@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>