board/sifive/hifive-unleashed: update rootfs offset to create a 64 MB image

As the rootfs size is set to 60M, let's put it right on the 4M offset
in the SD card, so that the generated sdcard.img can be exactly
64M. This will allow sdcard.img to be accepted as an image by Qemu,
which requires power of two sizes for the disk images.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/sifive/hifive-unleashed/genimage_sdcard.cfg

@@ -17,6 +17,7 @@ image sdcard.img {
 
 	partition rootfs {
 		image = "rootfs.ext4"
+		offset = 4076K
 		partition-type-uuid = 0fc63daf-8483-4772-8e79-3d69d8477de4
 		bootable = true
 	}