buildroot

configs: atmel: add sama5d3 xplained sd card image generation

Use genimage to generate a sd card with a fat partition (at91bootstrap,
u-boot, kernel, dtbs) and an ext4 partition (rootfs).

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>