distro: Add i686-linux port.

* distro/packages/bootstrap/i686-linux/bash,
  distro/packages/bootstrap/i686-linux/mkdir,
  distro/packages/bootstrap/i686-linux/tar,
  distro/packages/bootstrap/i686-linux/xz: New files.

* distro/packages/base.scm (%bootstrap-coreutils&co,
  %bootstrap-binutils, %bootstrap-glibc, %bootstrap-gcc): Add hashes of
  the i686-linux tarballs.

* Makefile.am (bootstrap_i686_linuxdir, dist_bootstrap_i686_linux_DATA,
  nodist_bootstrap_i686_linux_DATA): New variables
  (DISTCLEANFILES): Add $(nodist_bootstrap_i686_linux_DATA).
  (distro/packages/bootstrap/i686-linux/guile-bootstrap-2.0.6.tar.xz):
  New rule.