Add out-of-source U-Boot device tree support

Similaly to Linux, this patch adds the ability to copy in and build
out-of-source device tree sources during a U-Boot build.

Signed-off-by: James Balean <james@balean.com.au>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/uboot/Config.in

@@ -407,4 +407,18 @@ config BR2_TARGET_UBOOT_ENVIMAGE_REDUNDANT
 
 endif # BR2_TARGET_UBOOT_ENVIMAGE
 
+if BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG
+
+config BR2_TARGET_UBOOT_CUSTOM_DTS_PATH
+	string "Device Tree Source file paths"
+	help
+	  Space-separated list of paths to device tree source files
+	  that will be copied to arch/ARCH/dts/ before starting the
+	  build.
+
+	  To use this device tree source file, the U-Boot configuration
+	  file must refer to it.
+
+endif
+
 endif # BR2_TARGET_UBOOT

boot/uboot/uboot.mk

@@ -195,6 +195,9 @@ endef
 endif # BR2_TARGET_UBOOT_BUILD_SYSTEM_LEGACY
 
 define UBOOT_BUILD_CMDS
+	$(if $(BR2_TARGET_UBOOT_CUSTOM_DTS_PATH),
+		cp -f $(call qstrip,$(BR2_TARGET_UBOOT_CUSTOM_DTS_PATH)) $(@D)/arch/$(UBOOT_ARCH)/dts/
+	)
 	$(TARGET_CONFIGURE_OPTS) 	\
 		$(MAKE) -C $(@D) $(UBOOT_MAKE_OPTS) 		\
 		$(UBOOT_MAKE_TARGET)