gluon-web-private-wifi: add support for 802.11r

Add support for enabling IEEE 802.11r Fast BSS transitioning on WPA2-PSK
networks.

With PSK2 networks, the PMK can be derived from the PSK, thus all
information for generating is available to the AP without performing PMK
exchanges with other APs. Thus, we can offer a simple configuration to
potnetial users.

package/gluon-web-private-wifi/luasrc/lib/gluon/config-mode/model/admin/privatewifi.lua

@@ -1,4 +1,6 @@
 local uci = require("simple-uci").cursor()
+local unistd = require 'posix.unistd'
+
 local platform = require 'gluon.platform'
 local wireless = require 'gluon.wireless'
 
@@ -45,6 +47,12 @@ if platform.device_supports_mfp(uci) then
 end
 mfp.default = uci:get('wireless', primary_iface, 'ieee80211w') or "0"
 
+local ieee80211r = nil
+if unistd.access('/lib/gluon/features/wpa3') then
+	ieee80211r = s:option(Flag, "80211r", translate("Fast BSSID transition"))
+	ieee80211r:depends(encryption, "psk2")
+	ieee80211r.default = uci:get('wireless', primary_iface, 'ieee80211r') or false
+end
 
 function f:write()
 	wireless.foreach_radio(uci, function(radio, index)
@@ -67,6 +75,10 @@ function f:write()
 				disabled              = false,
 			})
 
+			if ieee80211r ~= nil then
+				uci:set('wireless', name, 'ieee80211r', ieee80211r.data)
+			end
+
 			-- hostapd-mini won't start in case 802.11w is configured
 			if platform.device_supports_mfp(uci) then
 				uci:set('wireless', name, 'ieee80211w', mfp.data)