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

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Stijn Tintel (stintel) 

Attached to Project - LEDE Project
Summary - netifd: implement point-to-point interface support
Task Type - Feature Request
Category - Packages
Status - New
Assigned To - 
Operating System - All
Severity - Low
Priority - Medium
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - 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