doc/usage: cmd-usage help file for askenv

help file for using askenv cmd is created.
It provides description on the command purpose,
description of arguments,
couple of examples (illustrating command usage),
configuration parameter and
possible return values.

Signed-off-by: Adarsh Babu Kalepalli <opensource.kab@gmail.com>
Add missing entry in doc/usage/index.rst.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/usage/askenv.rst

@@ -0,0 +1,87 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+askenv command
+===============
+
+Synopsis
+--------
+
+::
+
+    askenv name [message] [size]
+
+Description
+-----------
+
+Display message and get environment variable name of max size characters
+from stdin.
+
+name
+    name of the environment variable
+
+message
+    message is displayed while the command waits for the  value to be
+    entered from stdin.if no message is specified,a default message
+    "Please enter name:" will be displayed.
+
+size
+    maximum number of characters that will be stored in environment
+    variable name.this is in decimal number format (unlike in
+    other commands where size values are in hexa-decimal). Default
+    value of size is 1023 (CONFIG_SYS_CBSIZE - 1).
+
+Example
+-------
+
+Value of a environment variable env1 without message and size parameters:
+
+::
+
+    => askenv env1;echo $?
+    Please enter 'env1': val1
+    0
+    => printenv env1
+    env1=val1
+
+Value of a environment variable env2 with message and size parameters:
+
+::
+
+    => askenv env2 Please type-in a value for env2: 10;echo $?
+    Please type-in a value for env2: 1234567890123
+    0
+    => printenv env2
+    env2=1234567890
+
+Value of a environment variable env3 with size parameter only:
+
+::
+
+    => askenv env3 10;echo $?
+    Please enter 'env3': val3
+    0
+    => printenv env3
+    env3=val3
+
+Return Value of askenv command, when used without any other arguments:
+
+::
+
+    => askenv;echo $?
+    askenv - get environment variables from stdin
+
+    Usage:
+    askenv name [message] [size]
+        - display 'message' and get environment variable 'name' from stdin (max 'size' chars)
+    1
+
+Configuration
+-------------
+
+The askenv command is only available if CMD_ASKENV=y
+
+Return value
+------------
+
+The return value $? is set to 0 (true).
+If no other arguments are specified (along with askenv), it is set to 1 (false).

doc/usage/index.rst

@@ -17,6 +17,7 @@ Shell commands
    :maxdepth: 1
 
    addrmap
+   askenv
    base
    bootefi
    booti