[Packaging] Add a new way to build Tizen packages.

The new solution has two main benefits:

- It makes it possible to have incremental builds.
- It does away with the need to have a separate git tree and run the
  generate-flat-tree.sh script at all.

(Write detailed explanation of what has changed, what was wrong and what has
been fixed).

The generate-flat-tree.sh script and its instructions in the README will be
removed in a later commit, once we get more people to test the new solution.