SetupAssistant.app has many fun hidden dot files in /var/db/ and Pepijn Bruienne (@bruienne) has found another good one:
/var/db/.MBSkipWiFiSetupIfPossible This dot file will make SetupAssistant skip the WiFi setup portion of the setup in some cases. Only 10.11 and higher will respect this dot file.
For SetupAssistant to skip the WiFi setup
/var/db/.MBSkipWiFiSetupIfPossible, some criteria have to be met. The first is that the
/var/db/.MBSkipWiFiSetupIfPossible file must exist, this can be done by touching the file. The second is a WiFi network has to be added either as a profile, manually and then deleting the .AppleSetupDone file, or by
networksetup in a post image/first boot script. The end result is to have a network in the preferred networks list before SetupAssistant runs. Third is that when the Mac is booted, the WiFi network must be within range and provide the Mac with a valid IP address.
Note: This will not work with 802.1X WiFi networks that have user level authentication. I have not tested with with captive portal networks.