Fix issues with nd_light

The nd_light app did not interoperate with itself due to the way the link-layer address options of ND messages were handled. Also, a bug in the initialization caused next-hop resolution to be started only for one instance of nd_light in an app that uses multiple instances of it. Thanks to Max Rottenkolber for finding these issues.

The semantics of nd_light with respect to packets received from the north port has also been changed. The app now expects complete Ethernet frames and overwrites the Ethernet header with the header constructed from the address resolution process (see commit message for the rationale for this)