Netifs without link status support will keep sending discover
messages with increasing time between.
Netifs that support link status will wait for the link to be up,
and then start sending.
Netifs that support link status but send no link status events
will continue to not work (unless link status is up from the
first check when setting it up).
Set link state correctly in lwIP for interfaces that support
the NETOPT_LINK option. Interfaces that do not support it
(like tap for native arch) remain up all the time.
Netdevs that support NETOPT_LINK but do not send NETDEV_EVENT_LINK_UP/DOWN
events will end up with a mismatched link state - but DHCP would
already not start for them either.
Use netifapi to signal lwIP to do the work in its own thread.