Bluetooth: Mesh: Encapsulate feature config

Moves mesh feature configuration to a separate module, deprecating the
bt_mesh_cfg_srv structure. The initial values for the features should
now be enabled through KConfig, where new config entries have been added
for each feature.

This removes the upward dependency on the config server from the core
stack, and makes the config server a pure frontend for the configuration
states, as all spec mandated behavior around the feature states is now
encapsulated.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>