configs: bump raspberrypi_defconfig to use latest rpi-3.12.y

The Raspberry Pi people are now considering there stable version to
be based on the 3.12.series:
    http://www.raspberrypi.org/forums/viewtopic.php?f=29&t=75895



    > The 3.12 kernel tree has long term support and we are moving to it.

Bump the Raspberry Pi defconfig to use the latest stable version of
the rpi-3.12.y branch, based on linux-3.12.18.

Bump the kernel headers at the same time.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>