net: Always build the string_to_enetaddr() helper

Part of the env cleanup moved this out of the environment code and into
the net code. However, this helper is sometimes needed even when the net
stack isn't included.

Move the helper to lib/net_utils.c like it's similarly-purposed
string_to_ip(). Also rename the moved function to similar naming.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reported-by: Ondrej Jirman <megous@megous.com>