Make the xz2 dependency optional to allow building without a C cross
compiler. This allows clippy.sh to be used on e.g. macOS more easily:
```
./clippy.sh --target x86_64-unknown-linux-gnu --exclude-features xz2
```
The init module contains a small init system for running our integration
tests against a kernel. While we don't need a full-blown linux distro,
we do need some utilities.
Once such utility is `modprobe` which allows us to load kernel modules.
Rather than create a new module for this utility, I've instead
refactored `init` into `test-distro` which is a module that contains
multiple binaries.
The xtask code has been adjusted to ensure these binaries are inserted
into the correct places in our cpio archive, as well as bringing in the
kernel modules.
Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
