set fdtfile environment variable
- with our boot.scr this is a no-op because flash-kernel stores a
symlink from /dtb-5.18.0-4-arm64 to /dtbs/
{fk_kvers}/
{fdtfile} - with extlinux.conf under debian-installer only sets fdtdir to /dtbs/ and then expects u-boot to read /dtbs/freescale/imx8mq-mnt-reform2.dtb
- without this patch, u-boot will try reading /dtbs/imx8mq-MNT Reform 2.0.dtb which doesn't exist