Add usage and fix the help text of the command
Change-Id: I0e157d76f5d87bcda80f82a110912994908c8e03
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5023
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins