The idea is that package lists in data/package-lists/common/
will be used by all build flavours, while special individual
build flavours may have their own additional package lists
defined by configure options.
Split the 'iso' target into 'prepare' and 'iso' so it's possible to
debug the preparation stage without having to start an actual ISO build.
Add build and clean stages. Also add 'purge' target that "factory resets"
the build environment.
Add some tsun-tsun feel to the build log.
