arm: mvebu: Espressobin: Set environment variable fdtfile
Required for the generic distro mechanism.
Linux ships with 4 variants:
marvell/armada-3720-espressobin-v7-emmc.dtb
marvell/armada-3720-espressobin-v7.dtb
marvell/armada-3720-espressobin-emmc.dtb
marvell/armada-3720-espressobin.dtb
Use available information to determine the appropriate filename.
Fixes booting GRUB EFI arm64 on Fedora.
Reported-by: Dennis Gilmore <dennis@ausil.us> Signed-off-by: Andre Heider <a.heider@gmail.com>