Network stack cleanup: IPv4

[tbursztyka]

ipv4.c has been reworked already.
icmpv4.c should be reworked:
. all ipv4 headers must be filled-in through net_ipv4_create (get rid of setup_ipv4_header)
. icmpv4 code should be using macro (or enum) so it's more readable than hard-coded numbers
. net_pkt_ll_src lines 317/318 should not be set here (net_if already handles the case, and set's current iface's ll address)

declutter using #8723 rules