binman: Support packaging U-Boot for scenarios like OF_BOARD or OF_PRIOR_STAGE

For scenarios like OF_BOARD or OF_PRIOR_STAGE, no device tree blob is
provided in the U-Boot build phase hence the binman node information
is not available. In order to support such use case, a new Kconfig
option BINMAN_STANDALONE_FDT is introduced, to tell the build system
that a device tree blob containing binman node is explicitly required
when using binman to package U-Boot.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/dts/Kconfig b/dts/Kconfig
index 99ce75e..dabe008 100644
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -19,6 +19,24 @@
 	bool
 	select DTOC
 
+config BINMAN_STANDALONE_FDT
+	bool
+	depends on BINMAN
+	default y if OF_BOARD || OF_PRIOR_STAGE
+	help
+	  This option tells U-Boot build system that a standalone device tree
+	  source is explicitly required when using binman to package U-Boot.
+
+	  This is not necessary in a common scenario where a device tree source
+	  that contains the binman node is provided in the arch/<arch>/dts
+	  directory for a specific board. Such device tree sources are built for
+	  OF_SEPARATE or OF_EMBED. However for a scenario like the board device
+	  tree blob is not provided in the U-Boot build tree, but fed to U-Boot
+	  in the runtime, e.g.: in the OF_PRIOR_STAGE case that it is passed by
+	  a prior stage bootloader. For such scenario, a standalone device tree
+	  blob containing binman node to describe how to package U-Boot should
+	  be provided explicitly.
+
 menu "Device Tree Control"
 	depends on SUPPORT_OF_CONTROL