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/verdin-imx8mp_defconfig b/configs/verdin-imx8mp_defconfig
index 44fcfa1..a74033c 100644
--- a/configs/verdin-imx8mp_defconfig
+++ b/configs/verdin-imx8mp_defconfig
@@ -34,7 +34,7 @@
 CONFIG_OF_SYSTEM_SETUP=y
 CONFIG_BOOTDELAY=1
 CONFIG_USE_PREBOOT=y
-CONFIG_PREBOOT="setenv fdtfile imx8mp-verdin-${variant}-${fdt_board}.dtb"
+CONFIG_PREBOOT="test -n ${fdtfile} || setenv fdtfile imx8mp-verdin-${variant}-${fdt_board}.dtb"
 CONFIG_LOG=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_DISPLAY_BOARDINFO_LATE=y