Do not hard code /usr

Not all operation system install packages into /usr, make sure
create-tree.sh do respect the PREFIX variable

Change-Id: Iff358c9e22fb8969afaef2bb96e1f91eb7e91123
Reviewed-on: https://gerrit.libreoffice.org/9730
Tested-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Michael Stahl <mstahl@redhat.com>