buildroot

board: atmel: genimage: add 1M offset for FAT partition

With older SoCs and some SD cards, a 1024-bytes offset is needed to allow
ROM code to read the content of the FAT partition. As it is usual to get
a 1M bytes offset for the first partition, use this value to avoid boot
issues.

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