gnrc: use gnrc_pkt_search_type instead of LL_SEARCH_SCALAR

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

sys/net/gnrc/conn/gnrc_conn.c

sys/net/gnrc/network_layer/icmpv6/gnrc_icmpv6.c

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

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

sys/net/gnrc/network_layer/ndp/node/gnrc_ndp_node.c

sys/net/gnrc/network_layer/sixlowpan/gnrc_sixlowpan.c

sys/net/gnrc/network_layer/sixlowpan/nd/gnrc_sixlowpan_nd.c

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

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

sys/shell/commands/sc_icmpv6_echo.c