luci-base: add getWirelessDevices() rpc method

The getWirelessDevices() method merges the results of the
network.wireless/status call with corresponding per-radio and
per-network iwinfo data.

This allows to simplify the client side network state model
implementation and saves extraneous rpc roundtrips to fetch
iwinfo data after discovering the wireless devices.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>