Switch to use KERNELORG_MIRROR.
Use .xz tarball instead of bz2 as not all kernel mirrors carry .bz2 now.
This resolves various checkuri issues on the autobuilder infrastructure.
(From OE-Core rev: b2725bf1b2dbe69617801850659fcae7be0587b5)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>