JIM: document "echo" command

Document "-n" option in manual;
Modify "echo" command definition as COMMAND_HANDLER to
easily add help message
Add help message aligned with manual.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>

doc/openocd.texi

@@ -5446,9 +5446,10 @@ file (which is normally the server's standard output).
 @xref{Running}.
 @end deffn
 
-@deffn Command echo message
+@deffn Command echo [-n] message
 Logs a message at "user" priority.
 Output @var{message} to stdout.
+Option "-n" suppresses trailing newline.
 @example
 echo "Downloading kernel -- please wait"
 @end example