upl: Add a command

Add a 'upl' command to work with Universal Payload features. For now it
only supports reading and writing a handoff structure.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Kconfig b/boot/Kconfig
index d5f1304..e37b08b 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -747,6 +747,7 @@
 
 config UPL
 	bool "upl - Universal Payload Specification"
+	imply CMD_UPL
 	imply UPL_READ
 	imply UPL_WRITE
 	help