wireless: T6318: move country-code to a system wide configuration

Wireless devices are subject to regulations issued by authorities. For any
given AP or router, there will most likely be no case where one wireless NIC is
located in one country and another wireless NIC in the same device is located
in another country, resulting in different regulatory domains to apply to the
same box.

Currently, wireless regulatory domains in VyOS need to be configured per-NIC:

set interfaces wireless wlan0 country-code us

This leads to several side-effects:

• When operating multiple WiFi NICs, they all can have different regulatory domains configured which might offend legislation.
• Some NICs need additional entries to /etc/modprobe.d/cfg80211.conf to apply regulatory domain settings, such as: "options cfg80211 ieee80211_regdom=US" This is true for the Compex WLE600VX. This setting cannot be done per-interface.

Migrate the first found wireless module country-code from the wireless
interface CLI to: "system wireless country-code"

(cherry picked from commit 9e22ab6b2aee48029d3455f65880e45c558cf1da)