In case of tunnel over PPP(such as gretap over l2tp): tunnel interface
use PPP's peer address as remote address, netifd script will call
proto_add_host_dependency function, then netifd will search which device
can reach to the remote address. Before the patch, netifd don't consider
the PPP interface can reach to the remote address, so netifd will select
default route to remote address, it will lead to remote address unreachable.

Based on a patch by xiaofan <xfan1024@live.com>

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>