Edit: you don’t need to put Uboot on the SD card as @balbes150 says below. Just burn the LE image on the SD card then copy the boot.ini from balbes’ link. Disregard the following–
You can put Uboot on the SD card. The LE images possible have Uboot already in them (I’m not sure). If not, you can put it on any SD card using these instructionshttp://docs.khadas.com/bootcamp/CreateBurnCardViaCLI/. There is also a Windows method linked from that page.
Then you can write the Libreelec image onto the SD card after that.
How to make the Vim boot from the SD card I am still a bit confused about, but it has worked for me. I have had to use the MRegister method described here http://docs.khadas.com/develop/HowtoBootIntoUpgradeMode/ where you short 2 pads on the board marked M. But then other times it just worked with the SD inserted.
You can try flashing the dual boot Android/Ubuntu images from the firmware page onto eMMC using the Burning Tool. I’m not sure but I think the Uboot on these enables booting from SD card. I think holding down Function button while its booting works for some of the ROMs for booting from the SD card.
And then, as I said above, once you have booted from the SD card, you can choose “Install to emmc” from the shutdown menu of Libreelec itself and that should install bootloader and rom to eMMC. There was some talk in one of the Libreelec threads about having to copy a DTB file to the eMMC after installing, you might need to look into that.