rockchip: utilize CONFIG_DEFAULT_FDT_FILE

Currently the fdtfile environment variable is set to
CONFIG_DEFAULT_DEVICE_TREE which is Ñ–nternally used as U-Boot devicetree
source. The OS can use a different filename and Kconfig gives us the
ability to select a default devicetree via CONFIG_DEFAULT_FDT_FILE.
This also gives user configuring U-Boot via menuconfig the behaviour
someone would expect.

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Tested-By: Vagrant Cascadian <vagrant@debian.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/configs/chromebit_mickey_defconfig b/configs/chromebit_mickey_defconfig
index 3712177..b4d017c 100644
--- a/configs/chromebit_mickey_defconfig
+++ b/configs/chromebit_mickey_defconfig
@@ -13,6 +13,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="rk3288-veyron-mickey"
 CONFIG_DEBUG_UART=y
 # CONFIG_ANDROID_BOOT_IMAGE is not set
+CONFIG_DEFAULT_FDT_FILE="rk3288-veyron-mickey.dtb"
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_SPL_STACK_R=y