[FS#1037] netifd: implement point-to-point interface support

[LEDE Bugs]

For full information about what has changed in this task, visit the URL below and click the History tab.

FS#1037 - netifd: implement point-to-point interface support
User who did this: Stijn Tintel (stintel)
Task details edited:
-------
Several hosting providers, like Hetzner or OVH, assign IP addresses that in a different subnet than the gateway they provide. To properly support this, you need to configure the interface IP as point-to-point with iproute2, like this:

ip addr add x.x.x.x peer y.y.y.y dev ethX

>From IP-ADDRESS(8):

peer ADDRESS
       the address of the remote endpoint for pointopoint interfaces.  Again, the ADDRESS may be followed by a slash and a decimal number, encoding the network prefix length. If a peer address is specified,
       the local address cannot have a prefix length. The network prefix is associated with the peer rather than with the local address.


It is possible to work around this with static routes, which is what I have done currently, but it makes bird unhappy, logging this message every 20 seconds:
Tue Oct  3 12:25:19 2017 daemon.err bird4: KRT: Received route 0.0.0.0/0 with strange next-hop 176.31.101.254

See also [[http://bird.network.cz/pipermail/bird-users/2012-April/007696.html|this thread]] on the bird ML.

Please implement iproute2 point-to-point interface support in netifd.
-------

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1037