gluon-core: delete all network device sections (#2263)

Delete all default network device sections upon first boot.

Only LAN & WAN networks are defined at this point. We are using the
legacy way of definiting bridges via the interface sections ifname
option.

The prior filtering was based upon a single device and didn't take into
consideration that DSA interface names can be named arbitrarily.

Signed-off-by: David Bauer <mail@david-bauer.net>