Capture everything built before the repo system in a base tarball

Parts built before bash and the repo system are available aren't
stored in a clean repository tarball, so if any early file is
overwritten, it's lost. Fix this by creating a base.tar.bz2 right
after the repo is set up, to hold reference copies of early files.
This tarball isn't checksummed, since it varies considerably with
bootstrap options, but the binaries inside are protected by their
own checksums.

steps/improve/setup_repo.sh

@@ -5,3 +5,5 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 #
 mkdir -p /external/repo
+
+tar -cf - --exclude='/external/repo/*' --exclude='/external/repo-preseeded/*' --exclude='/external/distfiles/*' --exclude='/dev/*' --exclude='/proc/*' --exclude='/sys/*' --exclude='/tmp/*' / | bzip2 --best > /external/repo/base.tar.bz2