Workspace Management

[mbelluzzo]

Improve the way we use and maintain our workspace to avoid the extreme duplication of artifacts on the disk that we have today.

Also, improve the code in such a way that a "reusable" (BUILD_DIR) workspace isn't required while keep mixer to reuse the required artifacts (swupd key pair and update stream for the deltas) between builds.

This will have to be split in sub tasks, because the final solution isn't possible with current mixer requirements in place.