Added initial support for NDP (IPv6)

Added initial support for NDP (IPv6)

Following messages are now handled by PROX
- router_solicitation
- neighbour_solicitation
- router_advertisement
- neighbour_advertisement

The following parameters are supported (through the PROX config file)
- sub mode=ndp
	This will enable handling of router and neighbour solicitation
	and advertisement.
- local ipv6=xxxx:xxxx:xxxxx:xxxx:xxxx:xxxx:xxxx:xxxx
	This will configure the local IPv6 address of the port.
	This parameter is optional. If not specified, the local IPv6
	will be calculated from the EUI.
- global ipv6=xxxx:xxxx:xxxxx:xxxx:xxxx:xxxx:xxxx:xxxx
	This will configure the global IPv6 address of the port.
	This parameter is optional. If not specified, the global IPv6
	will be calculated from the EUI and the router prefix received
	from the router.
- ipv6 router=yes
	This will cause the core to behave as an IPv6 router
	i.e. it will generate Router Advertisement messages
	This is only useful in back to back cases, when no real
	IPv6 router is present in the setup.
- router prefix=xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx
	The router prefix usedin the router advertisement
	The prefix will be used by the node to build an IPv6 global
	address in cases none were configured.

"Unsollicited NA" parameter has been added within the core/task section.
If set to yes (Unsollicited NA=yes), then an unsollicited neighbour
Advertisement is sent at startup

A same core/task cannot support both l3 and ndp mode.

Those messages will be generated or handled when submode
is set to "ndp":
- neighbour sollicitation
- neighbour advertisement
- router sollicitation
- router advertisement

An example configuration is provided: config/ipv6.cfg in which
port 0 / core 1 plays the role of the generator and port 1 /
core 2 plays the role of the swap.

Change-Id: Id0ab32d384448b4cf767fb4a1c486fc023f4f395
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>