scripts: fix default work-dir name

It's been a long time the default work-dir changed its name
from 'target' to '.build'.

Change the left-over.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

reporter_name="Nye Liu"

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

reporter_comment=""