support/download: make hash file optional

Currently, specifying a hash file for our download wrapper is mandatory.

However, when we download a git, svn, bzr, hg or cvs tree, there's by
design no hash to check the download against.

Since we're going to have hash checking mandatory when a hash file
exists, this would break those downloads from a repository.

So, make specifying a hash file optional when calling our download
wrapper and bail out early from the check-hash script if no hash file is
specified.

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