Use bootstrap binaries that were generated with the `make-bootstrap'
changes introduced in previous commit.
* Makefile.am (nodist_bootstrap_x86_64_linux_DATA,
nodist_bootstrap_i686_linux_DATA): Change Guile tarball name.
(distro/packages/bootstrap/x86_64-linux/guile-2.0.7.tar.xz,
distro/packages/bootstrap/i686-linux/guile-2.0.7.tar.xz): New targets.
(distro/packages/bootstrap/x86_64-linux/guile-bootstrap-2.0.6.tar.xz,
distro/packages/bootstrap/i686-linux/guile-bootstrap-2.0.6.tar.xz):
Remove.
* build-aux/download.scm (file-name->uri): Use "20121219" directory.
* distro/packages/bootstrap.scm (%bootstrap-guile): Update file name.
(%bootstrap-coreutils&co, %bootstrap-binutils, %bootstrap-glibc,
%bootstrap-gcc): Update directory name and hashes.