interface: proto_ip: order by address index first

interface: proto_ip: order by address index first

At the moment, dnsmasq initscript generates dhcp-range for an interface
by inspecting first address of that interface from netifd ubus output.

Order by address index as specified in the uci config makes netifd ubus
output consistent with linux network interfaces' primary/secondary
address settings.  More importantly, the ubus output and dnsmasq config
generation will be more predictable.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Acked-by: Hans Dedecker <dedeckeh@gmail.com>