Configuration check should happen directly in the stack

In the actual implementation, the configuration check is done twice. The first check is done in the EplCnIf component when receiving the configuration packet. The second one is done when configuring the stack through its function API.
Stack should provide a new function API to check the configuration. This check will be used by the EplCnIf in case of Packet API based solutions and by the stack internally when configured through function API.