boot/barebox: do not check hash for custom versions

As we recently did for U-Boot, disable checking hashes for custom
versions of barebox.

Note that we currently have no hash file for barebox, but avoiding the
check will already be in place when we do add it.

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