Locally generated packet go through lookup/load-balance - locally-generated is an...

Locally generated packet go through lookup/load-balance - locally-generated is an attribute of the packet and checked where necessary

Add a flag to the vnet_buffer to indicate a packet is locally originated. Then in the rewrite nodes we can check this flag and not perform the TTL decrement. The switch path cost is expected to be fractions of a clock - the flags will be hot in the cache.
The cehcks are necessary to due the requirements that VPP must be able to emit an IP packet with TTL=255.

Change-Id: Ieb9cf06e34df54fd5c950293de8b665016295c51
Signed-off-by: Neale Ranns <nranns@cisco.com>