hostapd: Add Automatic Channel Selection (ACS) support

This adds ACS support to hostapd. Currently only survey-based
algorithm is available.

To use ACS you need to enable CONFIG_ACS=y in .config and use
channel=0 (or channel=acs_survey) in hostapd.conf.

For more details see wiki page [1] or comments in src/ap/acs.c.

[1]: http://wireless.kernel.org/en/users/Documentation/acs

Signed-hostap: Michal Kazior <michal.kazior@tieto.com>

src/ap/hostapd.h

@@ -310,6 +310,10 @@ struct hostapd_iface {
 	/* lowest observed noise floor in dBm */
 	s8 lowest_nf;
 
+#ifdef CONFIG_ACS
+	unsigned int acs_num_completed_scans;
+#endif /* CONFIG_ACS */
+
 	void (*scan_cb)(struct hostapd_iface *iface);
 };