Fix ipv6_defaultrouter

Summary:
When a FreeBSD server is set up with automatic
IPv6 network configuration, it included the
following entry in /etc/rc.conf.d/routing:

ipv6_defaultrouter="None"

Test Plan: Tested on WindRiver

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2109