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