Migrate network config into uci-defaults scripts

[stephen304]

Open to discussion but instead of importing network config file from each running device, it might be nicer to refactor the changes from default into a uci-defaults script. Maybe the script can do some checks for device-specific configs but if the modifications don't hurt anything we can apply it to all devices. Potential challenges are differing interface naming schemes (lanX for EB, eth0 for rpi4), device specific network config (WAN for rpi4). Potential solutions are adding a device specific uci-defaults in the device folder for any devices that need it (eg z-massmesh-device).

I want to do this because this would allow refactoring certain shared settings such as the meshnode LAN subnet and potentially mesh vlan config.