During ext SDK construction if we know we aren't including the full sstate for the image and SSTATE_MIRRORS isn't set in the local.conf file we've created (after we've generated the final local.conf), we should configure devtool to hide or disable the build-image and package subcommands since they won't be usable.

I'd like to do this in a manner that allows any devtool subcommand to be disabled from devtool's config file; this should let people disable parts of the ext SDK if they feel their users won't need them.