support/download: convert localfiles to use the wrapper

This drastically simplifies the localfiles 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 setting BUSYBOX_SITE = file:///tmp

 and running 'make busybox-source')
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>