initial-config: configure all wifi devices and remove set -u

We have to remove set -u because jshn.sh breaks with it.

This now configures all available Wi-Fi devices instead of just first one. This should cover case when user has device that supports only one specific band (2.4GHz for example) and the first Wi-Fi would be configured to other band (such as 5GHz). It also now reuses prepared configuration.

This time I properly fully tested it as part of medkit not just in running system. It starts the WiFi correctly and fixes failure because of jshn.sh.