net: ipv4: Specify default netmask for IPv4 addresses registered

When IPv4 address is added, currently the netmask remains empty. This
makes it impossible to use the address now that the netmask is
considered during address selection if the application does not
specify the netmask manually. Therefore specify a default netmask for
IPv4 addresses added to an interface with Kconfig.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>