infra/pkg-download: return just a list of URIs

infra/pkg-download: return just a list of URIs

Currently, the internal DOWNLOAD_URIS variable is set to be a list of
options to pass to the download wrapper, with each URI passed as
'-u URI'.

This precludes using that variable to get just the list of URIs, in
case we need to do something else with it.

Fix the variable to really only contain the list of URIs.

Adapt the caller accordingly.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>