ipv6_addr: remove ng_ prefix

Makefile.dep

examples/ng_networking/udp.c

sys/Makefile

sys/include/net/ipv6.h

sys/include/net/ipv6/addr.h

sys/include/net/ng_ipv6.h

sys/include/net/ng_ipv6/ext/rh.h

sys/include/net/ng_ipv6/hdr.h

sys/include/net/ng_ipv6/nc.h

sys/include/net/ng_ipv6/netif.h

sys/include/net/ng_ndp.h

sys/include/net/ng_ndp/internal.h

sys/include/net/ng_ndp/node.h

sys/include/net/ng_ndp/types.h

sys/include/net/ng_rpl/srh.h

sys/include/net/ng_sixlowpan/ctx.h

sys/include/net/ng_zep.h

sys/include/universal_address.h

sys/net/application_layer/ng_zep/ng_zep.c

sys/net/crosslayer/ng_pktdump/ng_pktdump.c