target: use list for target events

target: use list for target events

To simplify removing an event when it's set to an empty string,
switch event list from hardcoded simply linked list to helper's
double linked list.

While there, move the declaration of struct target_event_action
in 'target.c' as it is not anymore visible outside.

Change-Id: I799754c80055dc6d22db55aca483757e833714ff
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8813
Tested-by: jenkins