Provide Link up/down information using DrvEth events

Add support for the event ETH_EVENT_LINK_CHANGED in the low level. Since the HAL does not provide any indication for this, an internal timer has to be used to poll about the current Link status every 100ms.

Note: If the timer should not be fast enougth, the next solution would be to register for indications (interrupt) directly in the PHY controller.