phase1: don't enable rsync compression where unnecessary

phase1: don't enable rsync compression where unnecessary

Enabling rsync compression for already compressed files adds CPU
and memory overhead that negatively impacts upload speed on data
that cannot be further compressed.

By default rsync ignores '-z' on a number of files it knows to be
already compressed (based on filename suffix), but the list is not
exhaustive and doesn't include e.g. '.ipk'. Instead of trying to maintain
a list of all known compressed suffixes uploaded by the build system,
it's simpler and less error prone to simply disable the option for
specific steps where all or nearly all of the transferred files are known
to be already compressed.

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>