2 radvd specific tests with a number of utilities:
radvd to start RA daemon on specified interface
sniffer to collect icmp6 packets on specified interface
threaded version of sniffer

Change-Id: Icbdfa00bee5ba4628515f9a7ea460267eeaa70fe
Merge pull request #23 from kshileev/radvd

2 radvd tests