gluon-core: distribute dualband radios (#1606)

This commit distributes dualband radios evenly on 2.4 GHz and 5GHz with
2.4 GHz being prioritised higher than 5 GHz. This means in case a device
has only a single radio and this radio supports operation in both bands,
it will be set to 2.4 GHz.

Tested-by: Martin Weinelt <martin@darmstadt.freifunk.net>
Signed-off-by: David Bauer <mail@david-bauer.net>