add recovery option to allow installing previous GamerOS versions

[alkazar]

This is mostly for development. Sometimes when creating new test images, they fail to boot. This would make it easier to revert an existing GamerOS installation to a known good image.

1. Add a step before installation to select between "recovery" and "install" mode.
2. Selecting "install" should continue on with the installation as it is now.
3. Selecting "recovery" should allow you to choose from any available GamerOS image version and deploy it to the existing GamerOS install without reformatting the drive.
4. To prevent the automatic update immediately updating the system again upon restart, we should blacklist all known versions newer than the one selected. When a new image is published in the future, because it is not blacklisted, it would be installed by the updater.

One foreseeable problem is the different versions of frzr that are available on the different GamerOS versions. There may be some upgrade compatibility issues. Using an up to date install image to perform another recovery to manually upgrade to another version is probably an adequate work around.

Further thoughts:
For many (most?) users, they may not know what the previously installed version of GamerOS was. To make things easier for them, we could keep a record of what versions were installed and automatically select the previously installed version for users.

[alkazar]

No. 4 above is essentially already implemented. We can now specify specific version numbers to install, however, you cannot specify the build/hash number.