net: Give name to logging choice

This allows downstream modules to overwrite the default log level choice
using Kconfig.defconfig files.

For example, this becomes possible:

```
choice LWM2M_LOG_LEVEL_CHOICE
  default LWM2M_LOG_LEVEL_WRN
endchoice
```

In contrast to the configuration method, this then has an effect on all
applications stored in the downstream module.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>