[devices]: Fix arista-convertfs for aligning flash via sfdisk in Jessie (sonic-net#2402)

The sfdisk in Jessie use unit Cylinder by default. To perform 1M
aligning partition, 2048 sector unit is desirable instead.

Author: shawnli-van

files/initramfs-tools/arista-convertfs.j2

@@ -170,7 +170,7 @@ umount "$root_mnt"
 
 # Create a new partition table (content in flash_dev will be deleted)
 err_msg="Error: repartitioning $flash_dev failed"
-cmd="echo '2048' | sfdisk $flash_dev || (sleep 3; blockdev --rereadpt $flash_dev && fdisk -l $flash_dev | grep -q ${root_dev}.*Linux)"
+cmd="echo '2048' | sfdisk -u S --force $flash_dev || (sleep 3; blockdev --rereadpt $flash_dev && fdisk -l $flash_dev | grep -q ${root_dev}.*Linux)"
 run_cmd "$cmd" "$err_msg"
 
 sleep 5