base-files: generate "device" UCI type section for bridge

This switches from the old way of defining bridges in an "interface" UCI
section type (that should be used for layer 3 only). From now a defualt
board switch will have its own "device" UCI section type. It's a new &
preferred way of defining L2 devices.

Before:

config interface 'lan'
        option type 'bridge'
        option ifname 'lan1 lan2 lan3 lan4'
        option proto 'static'
        option ipaddr '192.168.1.1'
        option netmask '255.255.255.0'

After:

config device
        option name 'lan'
        option type 'bridge'
        list ports 'lan1'
        list ports 'lan2'
        list ports 'lan3'
        list ports 'lan4'

config interface 'lan'
        option ifname 'lan'
        option proto 'static'
        option ipaddr '192.168.1.1'
        option netmask '255.255.255.0'

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