xz: revert back to using a .bz2 archive

On machines where xzcat/unxz is not available, we build host-xz. So if
host-xz is itself downloaded as a xz-compressed archive, it doesn't
work. Revert back to a .bz2 archive.

Fixes:

  http://autobuild.buildroot.org/results/79e/79ecba46f353546ba60ae86dd3898b4d86c056a0/


  (and many similar failures)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>