apply non-free suite handling to buster release too

A release comprises of multiple suites, like: main, contrib, non-free etc, in Debian.

This change helps avoid having per suite branches. Because, otherwise, we'll have to train our downstreams to specially handle such packages. OTOH, this way, it remains transparent to the user

Signed-off-by: Ritesh Raj Sarraf ritesh.sarraf@collabora.com