support/download: convert svn to use the wrapper

This drastically simplifies the svn helper, as it no longer has to deal
with atomically saving the downloaded archive.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
(Tested by running 'make open2300-source')
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>