configs: bump kernel used for the RPi

The rpi-3.12.y branch has now been merged with the 3.12.26 upstream.

Bump kernel headers used for the toolchain at the same time.

Fixes
http://jenkins.free-electrons.com/job/buildroot/config=raspberrypi_defconfig/320/console



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>