bridge: rename "ifname" attribute to "ports"

bridge: rename "ifname" attribute to "ports"

Bridge aggregates multiple ports so use a more accurate name ("ports").
For backward compatibility add a temporary config translation.

Config example:

config interface 'lan'
        option type 'bridge'
        list ports 'lan1'
        list ports 'lan2'

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>