avahi: udhcpc default.script: only bring up avahi-autoipd on nak/timeout

The avahi udhcpc script used to start up avahi-autoipd at defconfig
(init) time, and then kill it if a dhcp lease was acquired, leading
to extra network traffic.

Change it to only start avahi-autoipd on timeout and dhcp nak.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>