An network devices that supports netdev_driver_t::get(NETOPT_LINK, ...)
also has to emit NETDEV_EVENT_LINK_UP and NETDEV_EVENT_LINK_DOWN with
lwip for IPv6 duplicate address detection to work. The background is
that the STM32 Ethernet MAC requires a periodic timer to poll for the
state to emit these events. For this reason, `stm32_eth_link_up` was
introduced to allow applications to select if they need these events.
With this dependency in place, IPv6 addresses won't get stuck in a
tentative state any more.
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