buildroot

configs/at91: refactor to use genimage.sh

Prefer the commom genimage.sh instead of a custom post-image.sh script.

Cc: Ludovic Desroches <ludovic.desroches@atmel.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>