netdev: use "relaxed" parsing

We parse each netdev file twice: once to determine the type and match conditions,
and then the second time properly. In bcde742e
the flags for the first parsing were (inadvertently I assume) were changed to
emit warnings. But this first pass is called with only [Match] and [NetDev] sections,
so we'd get warnings about all other section types. The obvious solution would be
to remove CONFIG_PARSE_WARN again, but I think it's better to keep the warnings
and set CONFIG_PARSE_RELAXED: we do want to get warnings about malformed lines and
such, and _RELAXED is enough to kill warnings about unknown sections.