infra/pkg-download: make the URI list a callable macro

infra/pkg-download: make the URI list a callable macro

Currently, that variable is context-dependent, as it expects the PKG
variable to exist and be defined to the current package.

This is not so clean, so change the variable to a callable macro.

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>