configs: bump kernel used for the RPi

The rpi-3.12.y branch has now been merged with the 3.12.20 upstream,
and contains some fixes for FIQ, v4l2, i2c, USB, and 1-wire GPIOs.

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

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