Revert "buildbot: switch buildbot & cros_sdk to xz"

This CL uploads a bzip2 compressed tarball with an xz extension.
Oops! Revert, so the CL can be fixed and resubmitted.

BUG=chromium-os:19287
TEST=unittests
Change-Id: I3c37751242d58e6db6b08d0c22d406151f3a060d
Reviewed-on: https://gerrit.chromium.org/gerrit/34821
Reviewed-by: Zdenek Behan <zbehan@chromium.org>
Tested-by: David James <davidjames@chromium.org>
diff --git a/scripts/cros_sdk.py b/scripts/cros_sdk.py
index 77ae559..8976014 100644
--- a/scripts/cros_sdk.py
+++ b/scripts/cros_sdk.py
@@ -21,7 +21,7 @@
 
 
 DEFAULT_URL = 'https://commondatastorage.googleapis.com/chromiumos-sdk'
-COMPRESSION_PREFERENCE = ('xz', 'bz2')
+COMPRESSION_PREFERENCE = ('bz2', 'xz')
 
 SRC_ROOT = os.path.realpath(constants.SOURCE_ROOT)
 OVERLAY_DIR = os.path.join(SRC_ROOT, 'src/third_party/chromiumos-overlay')
@@ -33,7 +33,7 @@
 ENTER_CHROOT = [os.path.join(SRC_ROOT, 'src/scripts/sdk_lib/enter_chroot.sh')]
 
 # We need these tools to run. Very common tools (tar,..) are ommited.
-NEEDED_TOOLS = ('curl', 'xz')
+NEEDED_TOOLS = ('curl',)
 
 
 def CheckPrerequisites(needed_tools):