support/download: properly catch missing hashes

When checking hashes reports no hash for a file, and this is treated as
an error (now: because BR2_ENFORCE_CHECK_HASH is set; later: because
that will be the new and only behaviour), exit promptly in error.

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>