upl: Add support for writing a upl handoff

Universal Payload provides a standard way of handing off control between
two firmware phases. Add support for writing the handoff information from
a structure.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Kconfig b/boot/Kconfig
index 3010629..d5f1304 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -748,6 +748,7 @@
 config UPL
 	bool "upl - Universal Payload Specification"
 	imply UPL_READ
+	imply UPL_WRITE
 	help
 	  Provides support for UPL payloads and handoff information. U-Boot
 	  supports generating and accepting handoff information. The mkimage
@@ -762,6 +763,13 @@
 	  which can be used elsewhere in U-Boot. This is just the reading
 	  implementation, useful for trying it out.
 
+config UPL_WRITE
+	bool "upl - Support writing a Universal Payload handoff"
+	help
+	  Provides support for encoding a UPL-format payload from a C structure
+	  so it can be passed to another program. This is just the writing
+	  implementation, useful for trying it out.
+
 endif  # UPL
 
 endif # BOOTSTD