buildroot

configs/raspberrypi*: update them to use genimage

Update the raspberrypi and raspberrypi2 configs to use genimage directly
to build the image.
Update the documentation to reflect this, and drop the volatile rootfs
option since it doesn't make much sense and it's not easily integrated
with the genimage configurations.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>