packaging: Fix adding the same files to the flat-tree archive

If we pass --update instead of --append to tar we only append to the archive
files that are newer than those already in the archive. This has the potencial
to make the archive generation faster in the incremental development case and
could also save some disk space.
packaging: Exclude src/out from the tar ball

If the same tree is used for development, src/out may contain the
results of previous builds, which are not needed for new builds.
Merge pull request #653 from vcgomes/packaging

[Packaging] Try to generate smaller flat-tree tar balls