net: lib: sockets: improve scalability through a hidden var

Introducing additional socket implementations is rather involved right
now due to some more or less convoluted code that had grown over time.

This change introduces an additional configuration variable in preparation
for additional socket API drivers. The idea is to reduce redundant code
and make existing code more readable by better exposing its actual intent.

Signed-off-by: Florian Grandel <jerico.dev@gmail.com>