helper/command: get current target from dedicated API

helper/command: get current target from dedicated API

Now that target override is uniformly implemented for all types of
commands, there is no need for target-prefixed "native" commands
(.jim_handler) to sneakily extract the overridden target from the
struct command.

Modify the commands to use the standard API get_current_target().

Change-Id: I732a09c3261e56524edd5217634fa409eb97a8c6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5666
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>