Added support for IMIX through config and command line

Added support for IMIX through config and command line

In Prox configuration IMIX can be specified using the following syntax:
imix=pkt_size,pkt_size... Up to 127 different pkt sizes can be specified.

Through command line, the following syntax can be used
imix core_id task_id pkt_size,pkt_size... Up to 127 packet sizes can be specified.

When IMIX is enabled, PROX will loop through the list of packet sizes.

PROX supports two different submodes to handle pcap files.
In the 1st submode (default, no submode specified), timestamps from
the pcap file are ignored. In this submode IMIX is supported as well and each
packets in the pcap file will be generated with all IMIX sizes.

In the second submode, called pcap in gen mode, packets are generated at
the timestamp specified in the pcap file. This submode does not support IMIX.

Change-Id: I53cbf1378a5364254285b81e6848350d98561184
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>