Don't abort when the .img.xz image generation fails, but give a warning instead.

The build runs fine on Raspberry Pi, but `xz` needs more memory than Pi provides.
Also keep the generated .img file needed later.
buildroot.sh: Build .img.bz2 as well

OS X users don't have `xz` installed by default, but it comes with `bzip2`.

The previous commit also helps with this scenario.