releng: Use baseline target file instead of zip for Oomph

releng: Use baseline target file instead of zip for Oomph

Instead of having to create a zip and pointing to it, use the
existing .target file. This requires to resolve the target first
then set it, as suggested here:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=500097#c7

Change-Id: I4e7b7d9b8ea8b013e24ba8c0ef760066c86251a2
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/77201
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>