Merge pull request #5118 from OlegHahm/udp_build_hdr_simplification

examples/gnrc_networking/udp.c

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

sys/include/net/gnrc/netreg.h

sys/include/net/gnrc/udp.h

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

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

sys/net/gnrc/conn/ip/gnrc_conn_ip.c

sys/net/gnrc/conn/udp/gnrc_conn_udp.c

sys/net/gnrc/netreg/gnrc_netreg.c

sys/net/gnrc/network_layer/icmpv6/echo/gnrc_icmpv6_echo.c

sys/net/gnrc/network_layer/ipv6/hdr/gnrc_ipv6_hdr.c

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

sys/net/gnrc/routing/rpl/gnrc_rpl_control_messages.c

sys/net/gnrc/transport_layer/udp/gnrc_udp.c

sys/shell/commands/sc_icmpv6_echo.c

tests/unittests/tests-gnrc_ipv6_hdr/tests-gnrc_ipv6_hdr.c