l2-rw: Packet rewrite feature for L2 bridges

l2-rw: Packet rewrite feature for L2 bridges

This patch defines a new l2input feature: l2-rw
It makes use of vnet_classify in order to match
packets and applies mask/value changes depending
on the matched classify entry.

Change-Id: Ia98c128931e59195bf3ecb66721e155ff9049a2e
Signed-off-by: Pierre Pfister <ppfister@cisco.com>