linux-headers: switch to xz format

The headers and kernels where changed to XZ format on commit
98b5cc3e, but the headers reverted back
to bz2 on the packaging of the toolchain.
This causes double kernel downloads when the versions match, so switch
back the headers to XZ.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>