distro: Use our own pre-built Guile to bootstrap.

* distro/packages/base.scm (%bootstrap-guile): Build from a tarball
  containing a pre-built Guile.

* distro.scm (%bootstrap-binaries-directory): New variable.
  (search-bootstrap-binary): New procedure.

* Makefile.am (bootstrapdir, bootstrap_x86_64_linuxdir,
  dist_bootstrap_x86_64_linux_DATA, DISTCLEANFILES, DOWNLOAD_FILE): New
  variables.
  (distro/packages/bootstrap/x86_64-linux/guile-bootstrap-2.0.6.tar.xz):
  New rule.
  (EXTRA_DIST): Add `build-aux/download.scm'.
  (.scm.go): Define the `DISTRO_BOOTSTRAP_DIRECTORY' and
  `DISTRO_INSTALLED_BOOTSTRAP_DIRECTORY' environment variables.

* pre-inst-env.in: Define `DISTRO_BOOTSTRAP_DIRECTORY'.

* build-aux/download.scm: New file.
* distro/packages/bootstrap/x86_64-linux/{bash, mkdir, tar, xz}: New
  files.