examples/ip_pipeline: measure CPU utilization

examples/ip_pipeline: measure CPU utilization

This patch adds CPU utilization measurement and idle cycle rate
computation to packet framework. The measurement is done by measuring
the cycles spent while a thread pulls zero packet from RX queue. These
cycles are treated as idle cycles (or headroom). A CLI command is added
to display idle cycle rate of specific thread. The CLI command format is
shown as following:

t <thread_id> headroom

Signed-off-by: Fan Zhang <roy.fan.zhang@intel.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>