support/download: warn when there's no .hash file

Instead of silently accepting a missing .hash file, print a warning.

This can be grepped from a build log, to find packages that still have
no hash, with the long-term goal of adding hashes for all packages.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>