buildroot

download: add missing '-d' option

The infrastructure needs to give the 'dl_dir' to the dl-wrapper which in its
turn needs to give it to the helper.  It will only be used by the 'git'
helper as of now.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>