If there is a hidden network in range it will have a 'nil' SSID value and the match function will result in an error aborting the updater.
This commit also checks for the bssid value so that if the bssid gathering whyever fails it doesn't let the whole updater fail.

/lib/gluon/upgrade/510-autoupdater-wifi-fallback:
- network config updated

/usr/lib/lua/autoupdater-wifi-fallback/util.lua:
- return radio, ssid and bssid

/usr/sbin/autoupdater-wifi-fallback:
- check if /sys/kernel/debug/batman_adv/bat0/gateways is readable
- update switch_to_fallback_mode() for own script /usr/sbin/wpa_supplicant
- improve log messages

/usr/sbin/wpa_supplicant: NEW
- script for executing iw dev <devname> connect [-w] <SSID> [<freq in MHz>] [<bssid>] [key 0:abcde d:1:6162636465]
- use bssid to specify the ap

determine the update branch with 'autoupdater-wifi-fallback -b <branch>'

/etc/config/autoupdater-wifi-fallback
  - just provide a config example

/lib/gluon/upgrade/510-autoupdater-wifi-fallback
  - if ther is no config, copy enable state from autoupdater
  - there is no need for setting the wireless config here
  - set a cron job to execute the wifi-fallback 10 minutes after the autoupdater

/usr/lib/lua/autoupdater-wifi-fallback/util.lua
  - improve get_available_wifi_networks()
    - use iwinfo, so there is no need to use io.poppen()
    - match the SSID to someting containing "Freifunk" (case-insensitive)
  - removed unused functions
    - get_site_macs()
    - is_in_fallback_mode()
    - neighbours()
    - interfaces()
    - get_wifi_neighbours()

/usr/lib/micron.d/autoupdater-wifi-fallback removed
  - the cron job is now prepared by /lib/gluon/upgrade/510-autoupdater-wifi-fallback

/usr/sbin/autoupdater-wifi-fallback
  - don't get min_update_secs from the config, just declare it here
  - remove the debug output
  - parse_args()
    - add new arg -b for feature use
    - add error message
  - preflight_check()
    - do not exit, just return true or false
    - do not check the fastd socket, it exists if fastd is running and does not belong to an established connection
  - connectivity_check()
    - was called check_connectivity() bevore, renamed to be similar to preflight_check()
    - do not check your wifi neighbours, they may have no connection too
    - some little code improvements
  - switch_to_falback_mode(wifiNetwork, radio)
    - we need the radio, too, to connect to the wifiNetwork
    - delete all wifi-ifaces, we will restore them later, don't worry
    - setup config for fallback wifi-iface
    - iw dev <dev> connect -w <ssid>, is not working any more, so we install wpa-suplicant-mini
    - revert the wireless directly after connecting to the AP
  - revert_to_standard_mode()
    - the config is allready restored, so just reinitialize the network
  - is_in_fallback_mode()
    - deleted
    - there is no reason to check this here
  - again no need to check the fastd socket
  - improve unreachable_since, execute os.time() only one time
  - as already mentioned, we need the corresponding radio, to connect to wifiNetwork

ffho-autoupdater-wifi-fallback-legacy: new
  - migration package to migrate from old version to the new one
  - deletes the obsolete config after an upgrade

  See https://github.com/freifunk-gluon/gluon/issues/455 for
  further details.

  If we change i.e. the mesh protocol or something else which
  breakes connectivity from mesh only nodes to their neighbours,
  they could get disconnected from the rest of the network.
  Therfore we try to update via the mesh network and if this fails,
  we try to find any open network to get our new firmware.

  This is also discussed in https://github.com/freifunk-gluon/gluon/issues/427