lib: do not build IPV4 code if not configured in

Although we love to build-test stuff and discard at link time, as soon
as we move tasks to be sections, we need to avoid building the file of
the task if not configured in.  This means lib/arp.c and lib/ipv4.c.

But, for consistency, avoid building icmp.c and bootp.c as well.
The alternative, which is ifconfig'ing the task itself results in
warnings like "'ipv4_init' defined but not used".
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>