Re-enable transitional artifact in the new artifact generator

In order for a user to be able to create a delta-artifact for a unit running an old version of mender (i.e., 2.1.x), with the old binary-delta client, and Mender-client (which does not fully support provides and depends), then unless the new generator supports writing this information into the meta-data section then the user would have to use an old version of the generator in order to update the device. Since this would be kinda cumbersome, the generator needs to support this, until all Mender versions without full provides and depends support are unsupported (i.e., the release of 2.5.x, or whatever version this will be).

Acceptance criteria:

• Re-enable transitional meta-data in the generator, and give it a command-line-option.

• Document this in Mender-docs