Corrected Wi-SUN certificate add and remove functions to work properly
with .json certificates. .json certificates are used only if certificates
added by add functions are not set. Corrected certificate add and remove
functions to work properly and improved function descriptions.
```
connectivity
├── netsocket
├── lwipstack
├── nanostack
│ ├── mbed_lib.json // nanostack-interface's mbed_lib.json
│ ├── include
│ │ └── nanostack // headers from features/netsocket/nanostack-interface
│ ├── source // sources from features/netsocket/nanostack-interface
│ ├── coap-service // used by Thread only - deprecate?
│ ├── mbed-mesh-api
│ ├── nanostack-hal-mbed-cmsis-rtos // HAL porting layer for Nanostack on mbed with CMSIS-RTOS
│ ├── sal-stack-nanostack
│ └── sal-stack-nanostack-eventloop
├── cellular // previously in features/cellular/framework/
├── ...
```
This PR is a part of a wider \connectivity refactoring and implements the above new directory structure for \nanostack.