Centralize indexes of private / public wifi ifaces in UCI

[jsha]

UCI is the unified config mechanism used in OpenWRT, upon which OpenWireless is based. We use UCI settings to configure the wireless networks. In order to configure the wireless networks we set configuration options named wireless.wifi-iface[1] and wireless.wifi-iface[2], corresponding to the public / private wifi.

Instead of relying on hardcoded indexes spread through the code, there should be exported variables in the common module name "public_wifi" and "private_wifi" that contain the above two interface names. Additionally, all code should be updated to refer to those variables rather than the configuration options directly.

Acceptance Criteria Given I have made these changes When I search for "wireless.wifi-iface[1]" in the code base Then There should only be one result, in the common module.