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>

package/kernel/mac80211/files/lib/netifd/wireless/mac80211.sh

@@ -29,7 +29,7 @@ drv_mac80211_init_device_config() {
 	config_add_string distance
 	config_add_int beacon_int chanbw frag rts
 	config_add_int rxantenna txantenna antenna_gain txpower
-	config_add_boolean noscan ht_coex acs_exclude_dfs
+	config_add_boolean noscan ht_coex acs_exclude_dfs background_radar
 	config_add_array ht_capab
 	config_add_array channels
 	config_add_array scan_list
@@ -273,6 +273,11 @@ mac80211_hostapd_setup_base() {
 			vht_center_seg0=$idx
 		;;
 	esac
+	[ "$band" = "5g" ] && {
+		json_get_vars background_radar:0
+
+		[ "$background_radar" -eq 1 ] && append base_cfg "enable_background_radar=1" "$N"
+	}
 	[ "$band" = "6g" ] && {
 		op_class=
 		case "$htmode" in