Choose xz compressed tarball for linux kernel.

Prefer xz compressed tarball so some bandwidth is saved for kernel headers
and kernel itself downloads.

Signed-off-by: Raúl Sánchez Siles <rasasi78@gmail.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>