Merge pull request #5498 from kaspar030/make_msg_content_ptr_void_star

core/include/msg.h

core/msg.c

drivers/nrf24l01p/nrf24l01p.c

drivers/pir/pir.c

pkg/emb6/contrib/conn/udp/emb6_conn_udp.c

pkg/lwip/contrib/netdev2/lwip_netdev2.c

sys/include/net/gnrc.h

sys/net/gnrc/application_layer/tftp/gnrc_tftp.c

sys/net/gnrc/application_layer/zep/gnrc_zep.c

sys/net/gnrc/conn/gnrc_conn.c

sys/net/gnrc/link_layer/netdev2/gnrc_netdev2.c

sys/net/gnrc/link_layer/nomac/gnrc_nomac.c

sys/net/gnrc/link_layer/slip/gnrc_slip.c

sys/net/gnrc/nettest/gnrc_nettest.c

sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c

sys/net/gnrc/network_layer/ipv6/nc/gnrc_ipv6_nc.c

sys/net/gnrc/network_layer/ipv6/netif/gnrc_ipv6_netif.c

sys/net/gnrc/network_layer/ndp/gnrc_ndp.c

sys/net/gnrc/network_layer/ndp/host/gnrc_ndp_host.c

sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c