bootstd: Add a simple command to list images

Add a new 'bootstd images' command, which lists the images which have
been loaded.

Update some existing tests to use it. Provide some documentation about
images in general and this command in particular.

Use a more realistic kernel command-line to make the test easier to
follow.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index cb7a23f..71970d5 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -40,6 +40,7 @@
    cmd/bootm
    cmd/bootmenu
    cmd/bootmeth
+   cmd/bootstd
    cmd/bootz
    cmd/button
    cmd/cat