[backend] Add support for *.livebuild projects

This adds support for Debian live-build 4.x based image builds. It requires
a build version with livebuild support.

Signed-off-by: Jan Blunck <jblunck@infradead.org>
[backend] Make livebuild type known to bs_repserver

This fixes the following error when doing osc local builds:
remote error: unknown repository type livebuild

Signed-off-by: Jan Blunck <jblunck@infradead.org>
[dist] require current build script for livebuild support

Signed-off-by: Jan Blunck <jblunck@infradead.org>