cmd: Add pause command

This command is being introduced with the goal of allowing user-friendly
"generic use case" U-Boot builds to pause until user input under some
situations.

The main use case would be when a boot failure happens, to pause until
the user has had time to acknowledge the current state.

Tested using:

    make && ./u-boot -v -T -c 'ut lib lib_test_hush_pause'

Signed-off-by: Samuel Dionne-Riel <samuel@dionne-riel.com>
Cc: Simon Glass <sjg@chromium.org>

doc/usage/cmd/pause.rst

@@ -0,0 +1,53 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later:
+
+pause command
+=============
+
+Synopsis
+--------
+
+::
+
+    pause [prompt]
+
+
+Description
+-----------
+
+The pause command delays execution waiting for any user input.
+
+It can accept a single parameter to change the prompt message.
+
+Examples
+--------
+
+Using with the default prompt:
+
+::
+
+    => pause
+    Press any key to continue...
+
+
+Using with a custom prompt:
+
+::
+
+    => pause 'Prompt for pause...'
+    Prompt for pause...
+
+Note that complex prompts require proper quoting:
+
+::
+
+    => pause Prompt for pause...
+    pause - delay until user input
+    
+    Usage:
+    pause [prompt] - Wait until users presses any key. [prompt] can be used to customize the message.
+
+Return value
+------------
+
+The return value $? is always set to 0 (true), unless invoked in an invalid
+manner.

doc/usage/index.rst

@@ -52,6 +52,7 @@ Shell commands
    cmd/mbr
    cmd/md
    cmd/mmc
+   cmd/pause
    cmd/pinmux
    cmd/printenv
    cmd/pstore