Enable BURST_MODE for l2fwd

Enable BURST_MODE for l2fwd

JIRA: VSPERF-267

This change adds optional burst mode to l2fwd module that makes use of
skb->xmit_more API available in Linux Kernel > 3.18 in order to batch
transmission of packets out of the NIC, increasing forwarding performance
significantly. By default burst mode is disabled. If a value greater than
1 is provided, burst mode is enabled to send that many packets at once.
Typical values would be burst=8 or burst=16.

Change-Id: I8ef5f86cf73d4cb5a8e4c618a86111ebf411dca8
Signed-off-by: Mesut Ali Ergin <mesut.a.ergin@intel.com>
Signed-off-by: Mallesh Koujalagi <malleshx.koujalagi@intel.com>