[OpenWRT] netmask issue on ipv4

[nemesifier]

In OpenWRT configurations, using the netmask config option is very common for IPv4 addresses and is also the preferred way to specify a netmask from the default web interface.

netjsonconfig doesn't include the netmask option but this can cause issues when a netmask config option is already present and netjsonconfig needs to override it.

Probably the best option is to include the netmask option for IPv4 addresses.

[nemesifier]

Closed by 4599383, which will be included in the next release shortly.