linux: do not check hashes for custom versions and tarballs

This one is a bit tricky, as the version can come from the linux-headers
package, so we must also account for that.

We currently have no hash file for linux, but better do the change now,
which allows us to later add a hash file.

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 <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>