support/download: add explicit no-hash support

support/download: add explicit no-hash support

Add support to explicitly state that an archive has no hash.

This can be used for archives downloaded from a repository, like a
git-clone or a subversion checkout, or using the github helper.

This will come in handy when we'll eventually make hashes mandatory as
soon as a .hash file exists: for some packages, like gcc, some versions
are downloaded as archives from upstream, while other versions may come
from a GitHub repository (via the github herlper).

In this case, a .hash file would exist, that contains hashes for the
downloaded tarballs, but archives downloaded from the repository would
not have a hash (since it is currently not possible to have reproducible
such archives). So, we'd need a way to explicitly state there is no
hash, on purpose, for those archives.

So, add 'none' as a new type of hash.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>