armbian-next: replace bare sync invocations by introducing wait_for_disk_sync(); sync early at start of build

- wait 10s for sync call; otherwise warn user to be patient
- wait_for_sync "reason for sync" everywhere
- add @TODO for /run/user/0 etc and XDG_RUNTIME_DIR in chroot

lib/functions/image/rootfs-to-image.sh

@@ -80,7 +80,7 @@ function create_image_from_sdcard_rootfs() {
 		run_host_command_logged sfdisk -l "${LOOP}" # @TODO: use asset..
 	fi
 
-	run_host_command_logged sync
+	wait_for_disk_sync "before umount MOUNT"
 
 	umount_chroot_recursive "${MOUNT}" "MOUNT"
 	[[ $CRYPTROOT_ENABLE == yes ]] && cryptsetup luksClose "$ROOT_MAPPER"