board/lego/ev3: Create images using genimage

This adds a script to create SD card and flash images for LEGO MINDSTORMS
EV3 using the genimage tool.

The default kernel config had to be modified to add support for squashfs
and to add a ram disk.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>