boot/uboot: do not check hash for custom versions

For custom tarballs, it's quite obvious we can not check hashes.

For custom versions, that's not so obvious. One might think we could
have hashes for all the official releases, but that's not possible: if
the user uses a released version of Buildroot (say 2015.05) much later
in the future, and wants to use the then-released 2038.02 U-Boot. It now
seems pretty obvious that Buildroot 2015.02 can't know the hash for
U-Boot 2038.02.

So, disable checking the hash for custom tarballs and custom versions.

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