First of all follow the wiki to get a build environment: Compile [LibreELEC.wiki] -> Stop at "Choose the PROJECT and ARCH to compile"
If you have done that, you should have a directory structure that is the same as the LE github page: GitHub - LibreELEC/LibreELEC.tv: Just enough OS for KODI
What jernej is trying to tell you is:
-> Add correct lines to scripts/uboot_helper: open the folder scripts and search the file uboot_helper.
example: LibreELEC.tv/uboot_helper at master · LibreELEC/LibreELEC.tv · GitHub
In this file you will see all supported allwinner boards. Add the lines for your board. The correct name should be the same of the linux dts file and adjust it to resemble the other lines in the file. Look here: linux/arch/arm64/boot/dts/allwinner at master · torvalds/linux · GitHub
-> You have to create H2 section and add your board. Then copy project/Allwinner/devices/H3 to H2. After that, you're on your own:
go to your start directoy and browse to the folder /projects/allwinner/devices/
example: LibreELEC.tv/projects/Allwinner/devices at master · LibreELEC/LibreELEC.tv · GitHub
Make a new folder for H2 and copy the stuff from inside H3 to the new H2 folder.
Proceed the wiki Compile [LibreELEC.wiki] starting from
"Choose the PROJECT and ARCH to compile" and use the allwinner H3 example as base. Should be something like this:
PROJECT=Allwinner ARCH=arm DEVICE=H2 UBOOT_SYSTEM=(your_board_name_from_uboot_helper) make image
No guarantee it will work so from there you are on your own.
If this is all to complicated, buy yourself a new board that is supported.