sixlowpan: move non-GNRC stuff to its own module

Makefile.dep

sys/Makefile

sys/include/net/ng_sixlowpan.h

sys/include/net/ng_sixlowpan/frag.h

sys/include/net/ng_sixlowpan/iphc.h

sys/include/net/sixlowpan.h

sys/net/crosslayer/ng_pktdump/ng_pktdump.c

sys/net/network_layer/ng_sixlowpan/frag/ng_sixlowpan_frag.c

sys/net/network_layer/ng_sixlowpan/frag/rbuf.c

sys/net/network_layer/ng_sixlowpan/iphc/ng_sixlowpan_iphc.c

sys/net/network_layer/ng_sixlowpan/ng_sixlowpan.c

sys/net/network_layer/sixlowpan/Makefile

sys/net/network_layer/sixlowpan/sixlowpan_print.c