luci-app-ffwizard-berlin: change static dns in wizzard

This static dns entry frei.funk becomes the min. ip of the dhcp
network aka the router ip. We use a named uci section with underscore instead of
dot because this fixes a syntax problem. We need a named section to access the
section through the wizzard.

Closes freifunk-berlin/firmware#16

@geirkairam could you have a look/test? Thanks!