build: Set owner root for all files on ubifs creation

When creating an image it makes no sense to have files owned by
the uid and gid on the host (and even creates a bug when for
example dropbear authorized_keys is included via env/files).
ar71xx: Add Netgear WNDR4300 2GHz led gpio and set it via uci-defaults.

The GPIO pin number is taken from Netgear's GPL sources. Other configuration
is basically copy-pasted from the 5GHz led config.