support/download: do not warn about missing hash file for repositories

When downloading from a repository, we explicitly pass no hash file,
because we can't check hashes in that case.

However, we're still printing a message that there is a missign hash
file.

Beside being a bit annoying (since we can't do anything about it), it
may also be wrong, especially for packages for which we support multiple
versions, with some being downloaded via a git clone and others as
tarballs.

Just print no warning when the path to the hash file is empty.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>