core/download: don't be needlessly verbose in backends

In 50c8b7e9 (support/download: support -q in all download backends), the
backend were made to respect the quietness of the main Makefile, when -s
is poassed on the 'make' command line. In doing so, they were all made
to be verbose by default.

However, the verbosity of some of the tools, like scp, is very high, and
is in fact intended for debug purposes.

Drop being verbose by default, just use whatever each tool deems normal
output. Only respect the quietness requested by the user.

Reported-by: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>