scripts: prevent trailing - in CT_TARGET

Not all target tuples consist of an VENDOR, KERNEL and SYSTEM part, build up the
tuple in such a way to no extra or trailing dashes are added to CT_TARGET

Signed-off-by: Bart vdr Meulen <bartvdrmeulen@gmail.com>

 reporter_name="Nye Liu"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-10/msg00046.html"

 reporter_comment=""