buildroot.sh: Indentation
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.
Added documentation for writing the image to SD card on a Mac.
Changed '.bz2' to '.img.bz2' since it's more correct and in line with the 'xz' description.
Replaced implicit link with an explicit markdown link.
Merge branch 'update-docs-for-bz2'