Adding a new field "destination_" including getter/setter to the ExtIfaceProcessor

Messages forwarded to the Wiselib by the external interface processor (ExtIfaceProcessor) elide the destination and just forward the source, length(of the payload), the payload itself and optionally Extended Data with locally calculated link metric. Without the destination transferred, the addressed nodes cannot distinguish between broadcast or unicast. Therefor I added the destination address id to the Extended Data.