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

board/boundary/nitrogen8m_som/nitrogen8m_som.c

@@ -328,6 +328,7 @@ static void set_env_vars(void)
 	printf("set_env_vars()");
 	env_set("board", "MNT Reform 2.0");
 	env_set("soc", "imx8mq");
+	env_set("fdtfile", "freescale/imx8mq-mnt-reform2.dtb");
 	env_set("imx_cpu", get_imx_type((get_cpu_rev() & 0xFF000) >> 12));
 	env_set("uboot_defconfig", CONFIG_DEFCONFIG);
 	env_set("stdin", "serial,usbkbd");