examples/l3fwd-graph: add graph config and main loop

Add graph creation, configuration logic and graph main loop.
This graph main loop is run on every slave lcore and calls
rte_graph_walk() to walk over lcore specific rte_graph.
Master core accumulates and prints graph walk stats of all the
lcore's graph's.

Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>