support/download/dl-wrapper: fix urlencode option never being sent to backend

support/download/dl-wrapper: fix urlencode option never being sent to backend

Since commit 38de434123 ("download: fix file:// BR2_PRIMARY_SITE
(download cache)"), the urlencode option is no longer passed to the
download backend, because we use ${backend} instead of
${backend_urlencode}.

We must get the urlencode information from backend_urlencode.

Signed-off-by: Damien Thébault <damien.thebault@vitec.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: rework commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>