We also make use of the boolean support of simple-uci to make scripts
clearer.

gluon-core: add get_mesh_devices(uconn) to gluon.util which will return all devices being up and having proto = gluon_mesh

Fixes #842

Some drivers (mt76) don't support arbitrary MAC addresses. Use the
addresses provided by the driver (avoiding the primary address) by default,
but fall back to our has-based scheme when the driver doesn't provide
(enough) addresses.