doc: sandbox: Add docs for the sb command

This command has a few small features, so document it.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/sb.rst

@@ -0,0 +1,54 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+.. index::
+   single: sbi (command)
+
+sbi command
+===========
+
+Synopsis
+--------
+
+::
+
+    sb handoff
+    sb state
+
+Description
+-----------
+
+The *sb* command is used to display information about sandbox's internal
+operation. See :doc:`/arch/sandbox/index` for more information.
+
+sb handoff
+~~~~~~~~~~
+
+This shows information about any handoff information received from SPL. If
+U-Boot is started from an SPL build, it shows a valid magic number.
+
+sb state
+~~~~~~~~
+
+This shows basic information about the sandbox state, currently just the
+command-line with which sandbox was started.
+
+Example
+-------
+
+This shows checking for the presence of SPL-handoff information. For this to
+work, ``u-boot-spl`` must be run, with build that enables ``CONFIG_SPL``, such
+as ``sandbox_spl``::
+
+    => sb handoff
+    SPL handoff magic 14f93c7b
+
+This shows output from the *sb state* subcommand::
+
+    => sb state
+    Arguments:
+    /tmp/b/sandbox/u-boot -D
+
+Configuration
+-------------
+
+The *sb handoff* command is only supported if CONFIG_HANDOFF is enabled.

doc/usage/index.rst

@@ -104,6 +104,7 @@ Shell commands
    cmd/reset
    cmd/rng
    cmd/saves
+   cmd/sb
    cmd/sbi
    cmd/scmi
    cmd/scp03