configs: add RaspberryPi defconfig

configs: add RaspberryPi defconfig

This is a default configuration for the RaspberryPi, tested and functional.

The kernel used is the one developped by the RaspberryPi foundation as
it's not fully supported currently in mainline kernel.  The configuration
used for the kernel is the default bcmrpi.

For more info, please find board/raspberrypi/readme.txt

[Peter: minor README tweaks, rename to readme.txt]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
[yann.morin.1998@free.fr: use correct kernel headers, update kernel cset]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>