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/doc/usage/index.rst b/doc/usage/index.rst
index 1f6518b..4246c06 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -114,6 +114,7 @@
    cmd/tftpput
    cmd/trace
    cmd/true
+   cmd/upl
    cmd/ums
    cmd/unbind
    cmd/ut