In BVI output node, set a signature value in packet buffer field
sw_if_index[VLIB_TX] so l2-input node can reliably check that
packet came into a BD through BVI so it can set the SHG of the
packet to 0 for a unicast packet.
Change-Id: I301aa2896677e11d0c964ca476dddcb5a8804fc2
Signed-off-by: John Lo <loj@cisco.com>