Throttle disk write to prevent etcd puking during upgrade

initramfs/rootfs-extra/init

@@ -235,6 +235,9 @@ fi
 if [ -z "$ROOT_DEV" ]; then
     ROOT_DEV="$(find_fallback_root_for_slot "$BOOT_PART" || true)"
     if [ -n "$ROOT_DEV" ]; then
+		if [ -z "$BOOT_PART" ]; then
+			BOOT_PART=A
+		fi
         log "Preferred root not found. Falling back to first valid root device: $ROOT_DEV"
     fi
 fi