- move steps from the single build script into Live Build "auto" scripts, per
  https://live-team.pages.debian.net/live-manual/html/live-manual/managing-a-configuration.en.html#325

- move config files into a separate src directory, so that the
  config tree can be removed without losing our config files,

- keep cache of Sugar source code separate to config tree, so that the
  config tree can be removed without losing the cache,

- completely remove the config and chroot and clean the build cache if
  the build script is re-run; helps with repeatability, (reported by
  Ibiam Chihurumnaya),

- report normal successful completion, as it is not clear if the build
  did succeed (reported by Ibiam Chihurumnaya),

- add Memorize activity.

Tested-by: James Cameron <quozl@laptop.org>
Signed-off-by: James Cameron <quozl@laptop.org>