scripts: in case of failed download, remove partial files

It happens from time to time that the server mis-behaves, and breaks the
connection right in the middle of nowhere, for no good reason, leaving us
with a partial file, on which the extract pass would choke.

Remove partial downloads, to fail early.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from eefacabd9e72f3d578e5dc5908026ed780f9a240)

reporter_name="Bart vdr. Meulen <bartvdrmeulen@gmail.com>"

reporter_url=""

reporter_comment="Config to build cross-compiler with mingw as kernel type"