Update generate-release.sh script to use rpi-image-builder tarball (#2235)
Update generate-release.sh script to use rpi-image-builder tarball (#2235)

* Update generate-release.sh script to use rpi-image-builder tarball

Also updated the wiki at:
https://github.com/mozilla-iot/wiki/wiki/How-To-Release-a-Gateway-OTA-Update

* Address review comments

mozilla-iot/gateway,moziot/gateway,mozilla-iot/gateway,mozilla-iot/gateway,mozilla-iot/gateway,mozilla-iot/gateway,moziot/gateway,moziot/gateway