you need mkimage which is part of the u-tools package.

Once your Internet Cube is no more able to boot, the repair process consists in configuring it for booting on the previous kernel that was working well. It will be necessary to make some modifications in the /boot, which is in a non crypted partition of the micro-SD card (the main partition beeing encrypted)

Put the micro-SD card in a GNU/linux computer. Open a console using root user (or use sudo command) and go in the /boot partition (size about 511MB, or 464Mo).

To start with, we'll have a look at previous kernel that are available:

# ls -al dtbs/

the resultat should be something like:

drwxr-xr-x 2 root root 1024 avril 22 01:55 4.1.0-2-armmp/
drwxr-xr-x 2 root root 1024 avril 22 09:41 4.5.0-1-armmp/

Here, the last good kernel is a 4.1.0-2 (which is the version of a fresh installed internet cube).

Copy the boot.src file:

# dd bs=1 skip=72 if=boot.scr of=boot.script

Then backup the boot.src on by renaming it:

# mv boot.scr boot.scr.bug

And edit boot.script file with your prefered editor (vi here):

# vi boot.script

Change the kernel version at line 44 with the last good one that you find previously:

:%s/4.5.0-1-armmp/4.1.0-2-armmp/g

Then make boot.src :

# mkimage -A arm -T script -C none -n "Debug boot.scr" -d boot.script  boot.scr

And finally create the symlink for dtb:

# ln -svf dtbs/4.1.0-2-armmp/sun7i-a20-olinuxino-lime.dtb dtb

Your Internet Cube is ready to start!

You just have to execute DoctorCube application that you can know find in YunoHost official applications in order to repair definitively your Internet Cube.