doc: describe command conitrace

Provide a man-page for the conitrace command.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

doc/usage/conitrace.rst

+conitrace command
+=================
+
+Synopsis
+--------
+
+::
+
+    conitrace
+
+Description
+-----------
+
+The conitrace command is used to test the correct function of the console input
+driver. It is especially valuable for checking the support for special keys like
+<F1> or <POS1>.
+
+To display escape sequences on a single line the output only advances to the
+next line after detecting a pause of a few milliseconds.
+
+The output is hexadecimal.
+
+Examples
+--------
+
+Entering keys <B><SHIFT-B><CTRL-B><X>
+
+::
+
+    => conitrace
+    Waiting for your input
+    To terminate type 'x'
+    62
+    42
+    02
+    =>
+
+Entering keys <F1><POS1><DEL><BACKSPACE><X>
+
+::
+
+    => conitrace
+    Waiting for your input
+    To terminate type 'x'
+    1b 4f 50
+    1b 5b 48
+    1b 5b 33 7e
+    7f
+    =>
+
+Configuration
+-------------
+
+The conitrace command is only available if CONFIG_CMD_CONITRACE=y.

doc/usage/index.rst

@@ -17,6 +17,7 @@ Shell commands
    bootefi
    bootmenu
    button
+   conitrace
    echo
    exit
    false