feat: Join an existing WiFi network

Add ssid and passphrase to config to configure the network to join. If
empty SSID is configured, it will create the "TonberryPicoAP..." network
in AP mode as before.

Holding the button 1 during startup will revert to AP mode regardless of
the current configuration.

Signed-off-by: Matthias Blankertz <matthias@blankertz.org>

software/frontend/index.html

@@ -384,6 +384,12 @@
             'root.LED_COUNT': {
                 'input-type': 'number'
             },
+            'root.WLAN.SSID': {
+                'input-type': 'text'
+            },
+            'root.WLAN.PASSPHRASE': {
+                'input-type': 'text'
+            }
         };
     
         function renderObject(obj, path) {