Documentation: add nsh boot command reference

Add boot commands.rst section describing its syntax and behavior,
including relative image path resolution against the NSH
current working directory, and boot command config.rst dependency-table entry
mapping CONFIG_BOARDCTL_BOOT_IMAGE and CONFIG_NSH_DISABLE_BOOT.

Signed-off-by: Junbo Zheng <zhengjunbo1@xiaomi.com>

Documentation/applications/nsh/commands.rst

@@ -143,6 +143,23 @@ default gateway.
 removing the preceding path segments and (optionally) removing any
 trailing ``<suffix>``.
 
+.. _cmdboot:
+
+``boot`` Boot an Application Image
+==================================
+
+**Command Syntax**::
+
+  boot [<image path> [<header size>]]
+
+**Synopsis**. Boot a new application firmware image by invoking
+``boardctl(BOARDIOC_BOOT_IMAGE)``. The ``<image path>`` may be
+absolute or relative; relative paths are resolved against the NSH
+current working directory, consistent with ``cp``, ``cat``, and
+``rm``. This command depends on ``CONFIG_BOARDCTL_BOOT_IMAGE``; if
+the boot image cannot be started, ``boardctl()`` returns and an
+error is reported.
+
 .. _cmdbreak:
 
 ``break`` Terminate a Loop

Documentation/applications/nsh/config.rst

@@ -35,6 +35,7 @@ Command                Depends on Configuration                    Can Be Disabl
 :ref:`cmdbase64enc`    ``CONFIG_NETUTILS_CODECS`` &&               ``CONFIG_NSH_DISABLE_BASE64ENC``
                        ``CONFIG_CODECS_BASE64``
 :ref:`cmdbasename`     .                                           ``CONFIG_NSH_DISABLE_BASENAME``
+:ref:`cmdboot`         ``CONFIG_BOARDCTL_BOOT_IMAGE``              ``CONFIG_NSH_DISABLE_BOOT``
 :ref:`cmdbreak`        ! ``CONFIG_NSH_DISABLESCRIPT`` &&           .
                        ! ``CONFIG_NSH_DISABLE_LOOPS``  
 :ref:`cmdcat`          ``CONFIG_NSH_DISABLE_CAT``                  .