configs: tdx: apalis/colibri_imx6: use preboot as well

Use PREBOOT as well. This allows a customer to just set fdt_board as
on any other module to customize the device tree for his carrier
board.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
diff --git a/configs/apalis_imx6_defconfig b/configs/apalis_imx6_defconfig
index 00c9a24..ea4ad27 100644
--- a/configs/apalis_imx6_defconfig
+++ b/configs/apalis_imx6_defconfig
@@ -29,6 +29,8 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_BOOTDELAY=1
 CONFIG_BOOTCOMMAND="run distro_bootcmd; usb start; setenv stdout serial,vidconsole; setenv stdin serial,usbkbd"
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="test -n ${fdtfile} || setenv fdtfile imx6q-apalis-${fdt_board}.dtb"
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_DISPLAY_BOARDINFO_LATE=y