Installing nRF Util when offline

For offline distribution of nRF Util and its installable commands, use the prepare-offline subcommand.

The following steps show how to install offline from a USB drive. It assumes Windows maps the USB drive to E:/.

When connected to the Internet, copy the nRF Util executable to the USB drive and run the following:
```
nrfutil prepare-offline E:/nrfutil-offline
```
This downloads and prepares tarballs containing the latest version of all available nRF Util commands to the USB drive, along with a portable package index that nRF Util uses for querying.
To install, for example, the device command on a host without an Internet connection, connect the USB drive and run the following:
```
nrfutil install device --from-offline E:/nrfutil-offline
```

When new versions are available, you can upgrade installed commands by running the following:

nrfutil upgrade --from-offline E:/nrfutil-offline