Option to override build date

Setting BUILD_DATE overrides the date used in the generated image names.
With this it is possible to have a consistent build date across all
architectures, making it easier to identify a release. Furthre more it
might be the key to have reproducible builds

Feature/2017.1.x

Merge branch 'feature/2017.1.x' of https://github.com/freifunk-stuttgart/site-ffs into feature/2017.1.x

gluon-web-mesh-vpn-fastd allows the user to switch between the security and performance VPN settions.

Feature/remove vpn schluessel ml hinweis

remove ibss

remove ibss mesh

Hinweis auf vpn-schlüssel-Mailingliste entfernt und durch Kontaktmöglichkeiten im Wiki ersetzt.

Hinweis auf vpn-schlüssel durch Hinweis auf Kontaktmöglichkeiten im Wiki ersetzt.

corrected Denglisch and Gateway to lowercase in nightly

+ update ffs-watchdog
+ update mesh ssid