Makefile: Add environment variable DEVICE_TREE to header

Makefile: Add environment variable DEVICE_TREE to header

Users have option to overwrite default device tree
(CONFIG_DEFAULT_DEVICE_TREE) via environment variable DEVICE_TREE.

Feature has been added long time ago by commit 74de8c9a1672
("dts/Makefile: Build the user specified dts") for a little bit different
reason.

But this variable can be also used for different purpose like choosing
proper configuration from FIT image in SPL.
And this is the functionality I would like to use on Xilinx Zynq devices
that current u-boot.img can be composed in the same way based on OF_LIST
and different configuration is taken based on platform specific SPL.
SPL requires low level ps7_init_gpl configuration that's why different
boards require different SPL with fixed board_fit_config_name_match().

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>