Tool chain instructions

[Johannes Kliemann]

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