boards/raspberrypi: update dt kernel sha1

The sha1 used for the DT enabled kernel is no longer available
(presumably, the rpi-3.18.y branch was rebased recently.) This updates
it to the lastest sha1 in the rpi-3.18.y branch.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>