command_registration: add empty usage field to chained commands

command_registration: add empty usage field to chained commands

Chained command require a subcommand as first argument. The usage
field for chained commands is not really important because the
"help" command will list all the subcommands with their respective
usage.

Add a empty usage field on all chained command.
The command "jlink config" can be either followed by a subcommand
or used alone, so use a dedicated usage string.

Change-Id: I43c3f8a766f96a9bdab4e709e3c90713be41fcef
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5017
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>