scripts: Add script to generate GCE image

* Upload disk tarball to Google Cloud Storage

   $ gsutil cp osv.tar.gz gs://google_cloud_storage_dir/osv.tar.gz

* Create an image which can be used to start instance

   $ gcutil addimage osv gs://google_cloud_storage_dir/osv.tar.gz

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
scripts: Add script to generate OSv VirtualBox Appliance

* Run scripts/gen-vbox-ova.sh

A build/release/osv.ova will be created

* Import OSv VirtualBox Appliance

Click File -> Import Appliance, Choose osv.ova to import

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>