Update gateway for IPv6 routing on WindRiver

Summary:
This gateway was discovered following this procedure:

• enable rt_adv on the interface
• ping google.com
• get an autoconfigured IP (different of the SLAAC expected one)
• ping again
• read the routing table

This procedure should be simplified and automated.

Ref T2017

Test Plan:
Deployed on WindRiver

Considered as working from the following statements:

• ping -6 google.com works
• From Eglide, able to SSH the server
• From Let's Encrypt servers, able to connect to nginx

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T2017

Differential Revision: https://devcentral.nasqueron.org/D3515