netifyd supports a '-F' filter option in 'bpf' notation to filter
packets from its consideration.

Add support for a uci 'filter' option. eg. filter to exclude SSDP
multicasts from a particularly noisy device:

option filter 'not (udp and dst 239.255.255.250 and dst port 1900 and src 192.168.1.5)'

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

Integrate with uci to support 'enabled/disabled' startup option and
override netify's default auto network selection.

config netifyd
       option enabled 0
       option autoconfig 1

autoconfig: Use netify's in-built interface selection
else use 'internal/external_if' definitions.

Also persist important agent config across sysupgrade
agent.uuid, serial.uuid, site.uuid

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

Signed-off-by: Darryl Sokoloski <darryl@sokoloski.ca>