gnrc_netif2: Introduction of a new GNRC network interface API

sys/include/net/gnrc/netif2.h

sys/include/net/gnrc/netif2/6lo.h

sys/include/net/gnrc/netif2/conf.h

sys/include/net/gnrc/netif2/ethernet.h

sys/include/net/gnrc/netif2/flags.h

sys/include/net/gnrc/netif2/internal.h

sys/include/net/gnrc/netif2/ipv6.h

sys/include/net/gnrc/netif2/mac.h

sys/include/net/netopt.h

sys/net/crosslayer/netopt/netopt.c

sys/net/gnrc/Makefile

sys/net/gnrc/netif2/Makefile

sys/net/gnrc/netif2/gnrc_netif2.c

sys/net/gnrc/netif2/gnrc_netif2_ethernet.c