buildroot.sh: Make building .img.xz optional

The build runs fine on Raspberry Pi, but `xz` needs more memory than Pi provides.
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.
Merge branches 'v1.1.x-set-e' and 'v1.1.x-img-bz2-2' into packt-2

Conflicts:
	clean.sh
.gitignore: Ignore autobuild artefacts
Merge branch 'v1.1.x-gitignore' into v1.1.x-autobuild-gitignore-merge

Conflicts:
	.gitignore
Merge branches 'v1.1.x-build-dir', 'v1.1.x-buildroot-minor', 'v1.1.x-autobuild-gitignore-merge' and 'v1.1.x-simplify-build' into packt-2
Merge branches 'v1.1.x-workaround-80-http', 'v1.1.x-misc' and 'v1.1.x-autobuild-debug' into packt-2
Merge branch 'v1.1.x-http-archive' into packt-2

Conflicts:
	update.sh
.gitignore: Unignore installer-config.txt
Add installer-config.txt