gluon-core: use OpenWrt label-mac as fallback (#2170)

This adds the OpenWrt label-mac device selection as the most preferred
fallback.

While this is only used on OpenWrt 19.07 for backports, we can also use
the label-mac device when backporting device support. This way, we have
to deal with less device-sepcific code downstream.

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