I installed grub, but replaced the grub.cfg with my own, just a simple menu. grub-mkconfig does not look for kernel8.img, so it will not generate a menu without making changes to the files in /etc/grub.d/. You can leave the default grub.cfg and just create a custom.cfg file with your own menu.
There is currently an issue with using uefi and a devicetree with a rpi kernel. It will boot when configuring the uefi with ACPI only. Anyway, doing the multibooting can be done, but it is not without issues as it all basically bleeding edge and things break. But that said, if you still want to give it a try, this might be helpful. If it does not boot, try downloading the latest uefi zip file, unzip it, and copy RPI_EFI.fd to your /dev/sda1 partition, overwriting the one installed. See the readme.md for info on configuring the new uefi.