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>
diff --git a/Makefile b/Makefile
index 0db44d4..c9ae31c 100644
--- a/Makefile
+++ b/Makefile
@@ -483,6 +483,7 @@
 version_h := include/generated/version_autogenerated.h
 timestamp_h := include/generated/timestamp_autogenerated.h
 defaultenv_h := include/generated/defaultenv_autogenerated.h
+dt_h := include/generated/dt.h
 
 no-dot-config-targets := clean clobber mrproper distclean \
 			 help %docs check% coccicheck \
@@ -1767,7 +1768,7 @@
 # prepare2 creates a makefile if using a separate output directory
 prepare2: prepare3 outputmakefile cfg
 
-prepare1: prepare2 $(version_h) $(timestamp_h) \
+prepare1: prepare2 $(version_h) $(timestamp_h) $(dt_h) \
                    include/config/auto.conf
 ifeq ($(wildcard $(LDSCRIPT)),)
 	@echo >&2 "  Could not find linker script."
@@ -1833,12 +1834,23 @@
 	 xxd -i ; echo ", 0x00" ; )
 endef
 
+define filechk_dt.h
+	(if test -n "$${DEVICE_TREE}"; then \
+		echo \#define DEVICE_TREE \"$(DEVICE_TREE)\"; \
+	else \
+		echo \#define DEVICE_TREE CONFIG_DEFAULT_DEVICE_TREE; \
+	fi)
+endef
+
 $(version_h): include/config/uboot.release FORCE
 	$(call filechk,version.h)
 
 $(timestamp_h): $(srctree)/Makefile FORCE
 	$(call filechk,timestamp.h)
 
+$(dt_h): $(srctree)/Makefile FORCE
+	$(call filechk,dt.h)
+
 $(defaultenv_h): $(CONFIG_DEFAULT_ENV_FILE:"%"=%) FORCE
 	$(call filechk,defaultenv.h)