buildroot

uboot: exclude from hash check except for latest version

Instead of excluding the hash specifically for the custom tarball and
custom version cases, exclude it always except in the one case where
we do have a hash: the latest version.

This simplifies the code a little because soon we will add hash checks
for git as well, so we also need an exclusion in that case.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>