[Add device] Provide KConfig options to configure added device

That could look like this:

```
# use a sec tag that can be safely overwritten
CONFIG_NRF_CLOUD_SEC_TAG=42
# hard-code client id
CONFIG_NRF_CLOUD_CLIENT_ID_SRC_COMPILE_TIME=y
CONFIG_NRF_CLOUD_CLIENT_ID="foo-bar"
```