spl: Add a way to specify a list of device trees to include

When building a FIT, more than one device tree can be included. The board
can select (at run-time) the one that it wants.

Add a Kconfig option to allow the list of devices trees (supported by the
board) to be specified.

When using SPL_LOAD_FIT, build u-boot.img in FIT format instead of the
legacy image format. Include all the listed device tree files in this FIT.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/dts/Kconfig b/dts/Kconfig
index df068a1..d585009 100644
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -59,6 +59,16 @@
 	  It can be overridden from the command line:
 	  $ make DEVICE_TREE=<device-tree-name>
 
+config OF_LIST
+	string "List of device tree files to include for DT control"
+	depends on SPL_LOAD_FIT
+	help
+	  This option specifies a list of device tree files to use for DT
+	  control. These will be packaged into a FIT. At run-time, SPL will
+	  select the correct DT to use by examining the hardware (e.g.
+	  reading a board ID value). This is a list of device tree files
+	  (without the directory or .dtb suffix) separated by <space>.
+
 config OF_SPL_REMOVE_PROPS
 	string "List of device tree properties to drop for SPL"
 	depends on SPL_OF_CONTROL