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>

board/atmel/sama5d2_xplained_mmc/genimage.cfg

@@ -23,6 +23,7 @@ image sdcard.img {
 		partition-type = 0xC
 		bootable = "true"
 		image = "boot.vfat"
+		offset = 1M
 	}
 
 	partition rootfs {

board/atmel/sama5d3_xplained_mmc/genimage.cfg

@@ -23,6 +23,7 @@ image sdcard.img {
 		partition-type = 0xC
 		bootable = "true"
 		image = "boot.vfat"
+		offset = 1M
 	}
 
 	partition rootfs {

board/atmel/sama5d4_xplained_mmc/genimage.cfg

@@ -24,6 +24,7 @@ image sdcard.img {
 		partition-type = 0xC
 		bootable = "true"
 		image = "boot.vfat"
+		offset = 1M
 	}
 
 	partition rootfs {