Install grub2 to ESP to boot to mmcblk0

[Toerless Eckert]

Notebook with one nvme with EFI: ESP and vendor OS. Would like to add linux to a
separate disk, but only option is mmcblk0 and PC bios can not boot from mmcblk0.

Trying to add grub2 to ESP and make it boot to linux on mmcblk0, but could not
find enough documentation how to make it work.

What i tried was to install debian onto mmcblk0p1, chroot into it, 
mounted ESP onto /boot/efi, and ran grub-install --removable. This
created grubx64.efi in ESP, but when booting to it, i only got into grub rescue.

Q: Anything that i could do in the grub rescue shell to figure out why
its not working ? ist storage devices found or the like ?

Q: Does grubx64.efi only rely on PC BIOS to find and read storage
devices, or does it have the option to include its own drivers (like a
linux kernel) to access storage type like mmcblk0 ?

Q: Any suggestions how i could make this work ?

Thanks