Improve mechanism for using loopback interface as BVI for BDs

Improve mechanism for using loopback interface as BVI for BDs

When loopback interface is configured as BVI, instead of changing its
output node from loopN-output to l2-input, the loopN-output node is now
kept while its next tx node is changed from ethernet-input to l2-input.
The packet setup previously done in bvi_to_l2 as part of l2-input is now
performed in the loop output node.

This change adds an extra node in the BVI output path but provides the
following improvements:
1. IP address/route created on loopback prior to it being configured as
   BVI will still work properly. The requirement to (re)configure IP/route
   on loopback after it is configured as BVI is removed.
2. The output stats for loopback interfaces are always provided irrespective
   of their BVI configuration.
3. The loopback-BVI output stats can be batch updated outside the packet
   loop in output node, instead of per packet update in l2-input node,
   making l2-input node more efficient for BVI packets.
4. Restore original node property as implemented in node.c function
   vlib_node_add_next_with_slot() where next node indices stored in next
   slots of each node will remain unique.
5. Packet trace for BVI output includes loopN output node which provides
   useful packet data.

Change-Id: I7f5bc72ef953a367363a179088210596881f9e73
Signed-off-by: John Lo <[email protected]>