Following feature ACD also needs a random IPv4 address as fallback. This patch
moves the function ipv4ll_random_ip() from gdhcp/ipv4ll.h to src/shared/arp.h.
The gdhcp/ipv4ll.h is now empty and can be removed. The function arp_random_ip
uses __connman_util_get_random() from src/util.c. In Makefile.am this patch
removes arp.c from target netlink-test to avoid depenencies of arp.c
(src/util.c, src/log.c and src/backtrace.c).