rockchip: utilize CONFIG_DEFAULT_FDT_FILE

rockchip: utilize CONFIG_DEFAULT_FDT_FILE

Currently the fdtfile environment variable is set to
CONFIG_DEFAULT_DEVICE_TREE which is іnternally 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>