New features

• Export list of extensions to an environment variable.
• Require extension version and include it in extension names for images.
• Add license file.

Improvements

• Each extension sysroot now maintains its own update-alternatives database.
• Update extension DNF to execute scriptlets.
• Enable scriptlets on extension install for update-alternatives support.
• Update avocado init configs with default extension version string.
• Update runtime enable from runtime to os-release.