updates-kernel: Adds option to share multiple directories (#33)

- Adds shared-dirs parameter that has an option to handle
 multiple directories and demanded rootfs location (for example /opt)
- shared-dir is now deprecated and it is a link to shared-dirs
- Adds rootfs-size parameter to change size of the rootfs image, it
 also has `auto` option (default) to let the action to choose size
 automatically
- Deletes one path to store user data on host, now there are two special
 catalogs: pip and repos to store python packages and github repos
- Changes default user catalog on target from `/root` to `/home`
- Deltes paration for user data, now everything is stored directly on
root
- Action refactoring
- Updates readme
- Adds option to disable network in the emuated Linux