Add support for the rpi-0, which is basically a rpi (model A+) in a
smaller form-factor.

This one does not have an ethernet port, so we just remove it from the
configuration (or we could use the existing rasbperrypi_defconfig and
suffer from a longer boot time because of the waiting for eth0).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>