fff-hoods/fff-wireless: Reconfigure instead of delete and create

Previously, when configurehood switched status, all WiFi devices
were completely rewritten and all interfaces were deleted and
recreated. This is both unnecessary and ugly.

This patch redesigns WiFi setup to create all interfaces (in
parallel) initially, and then only to enable/disable them as
necessary. Where reconfiguration is necessary, only the variable
parts are changed.

Since most of the wifi-device config is already created by
OpenWrt, this builds based on the existing wifi-devices and
only removes the default wifi-ifaces.

This patch will not change the logic (codeflow) of configurehood,
but only affects how action on the WiFi devices/interfaces is
taken.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>