need to skip binding dev to ipip tunnel when node's advertising address is on loopback interface

[tamihiro]

Still working on the same scenario as 778, I've got gobgp to be able to establish peers with my patch #777. However nodes are still unreachable to each other through the ipip tunnel interface. I have recognized network_routes_controller.go creates ipip tunnel with device name set to nrc.nodeInterface, which prevents the tunnel from working when tunnel source address is on loopback interface.

Below is an example of non-working tunnel.

8: tun-192168014@lo: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 65496 qdisc noqueue state UNKNOWN group default qlen 1000

I have submtted another PR #779. With this patch ipip tunnel is created without being bound to loopback interface, like below.

8: tun-192168014@NONE: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1460 qdisc noqueue state UNKNOWN group default qlen 1000

I'd appreciate your review on this one in addition to my previous PRs.