wifi.lua: Add support for toggling WPS button.

The patch checks the existence of the needed files for the WPS support and if they are present, shows the option to toggle WPS pushbutton settings.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
Merge pull request #219 from hnyman/WPS-toggle

wifi.lua: Add support for toggling WPS button