Add support for vdev tap devices

Add support for vdev tap devices

This commit adds support for the creation of tap devices.
To create a tap device, and associate it with a DPDK port,
add within the port section:
vdev=<tap_name>
local ipv4=<ip_assigned_to_port>
<tap_name> will appear as a kernel network device, with an IP <ip_assigned_to_port>

ARP packets received from the network on the DPDK interface will be forwarded to the kernel.
1st packet which needs to be generated to a new IP will cause an IP packet
to be sent to the kernel (using socket sendto) forcing the kernel to send an ARP_REQUEST.

In this commit, there is no notification from the kernel about ARP table changes.
This is subject to further commits.

Two config files, gen_tap.cfg and swap_tap.cfg have been created. They work one against each other.
*** Note however that they do not work if running within the same host
(with the port being connected back to back) ***
This is due to the fact that the kernel would bypass the ports and use local interface...

Change-Id: Iadeec0d99e3c693472ea44bdb9163a3bf97df2fa
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>