I have used iface_eth* settings in my /etc/conf.d/net since 2005.0, using docs valid at the time These settings are no longer documented in net.example, so I assume they are deprecated and replaced with config_eth*. The baselayout ebuild does not check for use of iface_eth* and does not supply warnings about them either.
Please attach your full /etc/conf.d/net file as baselayout does use iface_eth0 and others provided that you aren't using config_eth0