ng_ndp: prepare for parallel NDP implementations

Makefile.dep

sys/Makefile

sys/include/net/fib.h

sys/include/net/ng_ndp.h

sys/include/net/ng_ndp/internal.h

sys/include/net/ng_ndp/node.h

sys/net/network_layer/ng_ipv6/ng_ipv6.c

sys/net/network_layer/ng_ndp/internal/Makefile

sys/net/network_layer/ng_ndp/internal/ng_ndp_internal.c

sys/net/network_layer/ng_ndp/ng_ndp.c

sys/net/network_layer/ng_ndp/node/Makefile

sys/net/network_layer/ng_ndp/node/ng_ndp_node.c