Expose the autoupdate settings to package-builder

[marpaia]

Similarly to how you can call package-builder make with --insecure and the resultant launcher command will be ran with --insecure, we need to expose setting (or disabling) the --autoupdate flag as well. Right now, since --autoupdate defaults to false, we need this flag to create packages which have autoupdate enabled, but in the future, someone might want to use this to disable autoupdate if it becomes the default.

Additionally, pending the completion of #155, the update channel should also be configurable (with a default of "stable").