hostapd: introduce background_radar option

hostapd: introduce background_radar option

Introduce a new option background_radar to toggle hostapd's background
radar feature. Enabling this allows DFS CAC to run on dedicated radio RF
chains while the radio(s) are otherwise running normal AP activities on
other channels.

As OpenWrt configures hostapd to use a channel list even when a single
channel is configured, using this feature requires a list of channels in
/etc/config/wireless. Alternatively, channel can be set to auto.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: David Bauer <mail@david-bauer.net>