dts: Add support for adding DT overlays in u-boot.img

If u-boot.img is a FIT image, CONFIG_OF_OVERLAY_LIST can be used to add
DT overlays to u-boot.img.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/dts/Kconfig b/dts/Kconfig
index 2bd959a..64c98dd 100644
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -130,6 +130,14 @@
 	  device tree files (without the directory or .dtb suffix)
 	  separated by <space>.
 
+config OF_OVERLAY_LIST
+	string "List of device tree overlays to include for DT control"
+	depends on SPL_LOAD_FIT_APPLY_OVERLAY
+	help
+	  This option specifies a list of device tree overlays to use for DT
+	  control. This option can then be used by a FIT generator to include
+	  the overlays in the FIT image.
+
 choice
 	prompt "OF LIST compression"
 	depends on MULTI_DTB_FIT