build log: add message when starting target-finalize target

The target-finalize target does a large number of actions (removing
unnecessary files, stripping objects, etc.) but does not have a header. This
makes it seem that all these actions are done as part of the last action
before target-finalize, for example:
    >>> makedevs undefined Installing to target

To make a clear distinction, add a message to the beginning of
target-finalize:
    >>>   Finalizing target directory

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>