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>

doc/develop/bootstd/overview.rst

@@ -453,7 +453,7 @@ drivers are bound automatically.
 Command interface
 -----------------
 
-Three commands are available:
+Four commands are available:
 
 `bootdev`
     Allows listing of available bootdevs, selecting a particular one and
@@ -468,6 +468,25 @@ Three commands are available:
     Allow listing of available bootmethds, setting the order in which they are
     tried and bootmeth specific configuration. See :doc:`/usage/cmd/bootmeth`
 
+`bootstd`
+    Allow access to standard boot itself, so far only for listing images across
+    all bootflows. See :doc:`/usage/cmd/bootstd`
+
+Images
+------
+
+Standard boot keeps track of images which can or have been loaded. These are
+kept in a list attached to each bootflow. They can be listed using the
+``bootstd images`` command (see :doc:`/usage/cmd/bootstd`).
+
+For now most bootmeths load their images when scanning. Over time, some may
+adjust to load them only when needed, but in this case the images will still
+be visible.
+
+Once a bootflow has been selected, images for those that are not selected can
+potentially be dropped from the memory map. For now, this is not implemented.
+
+
 .. _BootflowStates:
 
 Bootflow states