configs: tdx: Do not overwrite fdtfile if it got set manually

In case a customer wants to set fdtfile currently preboot overrides it
always with preboot just before the bootdelay. Use test -n to check
if fdtfile is already set and only set it if nothing got touched manually

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
diff --git a/configs/colibri-imx6ull_defconfig b/configs/colibri-imx6ull_defconfig
index 3ef7a6a..2fba7aa 100644
--- a/configs/colibri-imx6ull_defconfig
+++ b/configs/colibri-imx6ull_defconfig
@@ -18,7 +18,7 @@
 CONFIG_BOOTDELAY=1
 CONFIG_BOOTCOMMAND="run ubiboot || run distro_bootcmd;"
 CONFIG_USE_PREBOOT=y
-CONFIG_PREBOOT="setenv fdtfile imx6ull-colibri${variant}-${fdt_board}.dtb"
+CONFIG_PREBOOT="test -n ${fdtfile} || setenv fdtfile imx6ull-colibri${variant}-${fdt_board}.dtb"
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_DISPLAY_BOARDINFO is not set