Tool chain instructions

Hi,

with the new toolchain, the compression has changed from bzip2 to xz. The toolchain instructions [1] still state to use

sudo tar xPfj genode-toolchain-<version>-<arch>.tar.bz2

This doesn't work anymore as `-j` requires bzip2 and fails with the following error:

bzip2: (stdin) is not a bzip2 file. tar: Child returned status 2 tar: Error is not recoverable: exiting now

A fix would be omitting `-j`, resulting in

sudo tar xPf genode-toolchain-<version>-<arch>.tar.xz

This is also safe since current versions of tar extract bzip2 without the usage of `-j`.

Regards, JK

[1]: https://genode.org/download/tool-chain