Thank you @Carlos Ervedoso, it works.
My fault I didn't follow with trust @jock's detailed installation manual.
When I tried for the first time armbian and librelec images on h20 box, none of them was booting. Only the multitool worked. But after resetting the eMMC, even the multitool was no longer booting.
The Armbian image still seemed not to boot (due to the absence of LEDs and HDMI of the default dtb; at that time I had not yet connections to the serial interface) so I (erroneously) deduced that for the h20 box the eMMC reset was a wrong step to take. My fault.
Now it works fine: thanks again for your great job and for sharing it.