Mounting steps doesn't work for me
mount -o loop
I have replaced for these kpartx commands:
kpartx -v -a synoboot/synoboot.img
mount /dev/mapper/loop13p1 usb/part1
mount /dev/mapper/loop13p2 usb/part2
and after umount commands:
sudo kpartx -d -v synoboot/synoboot.img