base-files: use cidr for config_generate

base-files: use cidr for config_generate

Generate network configuration replacing netmask with CIDR.

Depends on:
https://github.com/openwrt/openwrt/pull/13765

Using CIDR provides the following advantages:
* Consolidate notation for IPv4 and IPv6 addresses.
* Consolidate notation for IP addresses and routing targets.
* Simplify network configuration and troubleshooting.
* Follow the transition from net-tools to iproute2.

Resulting configuration example:
```
config interface 'loopback'
	option device 'lo'
	option proto 'static'
	list ipaddr '127.0.0.1/8'

config interface 'lan'
	option device 'br-lan'
	option proto 'static'
	list ipaddr '192.168.1.1/24'
```

Signed-off-by: Vladislav Grigoryev <vg.aetera@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/13780
Signed-off-by: Robert Marko <robimarko@gmail.com>