the mbed-lpc1768 according to  doesn't have any network interfaces as far as I see. Because of this there are no network drivers implemented and ifconfig doesn't have anything to show. Do you use it with any kind of extension board?
as Hauke pointed out already, RIOT supports Ethernet devices in general but still needs support for the specific hardware used. For your device this would mean implementing the interfaces defined in netdev2.h  and netdev2_eth.h .
If you are willing to take on this task we're happy to support you with any questions arising and you can take a look how it is done for other existing Ethernet devices like the enc28j60 .