configs: bump kernel used for the RPi

The rpi-3.12.y branch has now been merged with the 3.12.25 upstream.

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

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>