MINOR: event_hdl: normal tasks support for advanced async mode

MINOR: event_hdl: normal tasks support for advanced async mode

advanced async mode (EVENT_HDL_ASYNC_TASK) provided full support for
custom tasklets registration.

Due to the similarities between tasks and tasklets, it may be useful
to use the advanced mode with an existing task (not a tasklet).
While the API did not explicitly disallow this usage, things would
get bad if we try to wakeup a task using tasklet_wakeup() for notifying
the task about new events.

To make the API support both custom tasks and tasklets, we use the
TASK_IS_TASKLET() macro to call the proper waking function depending
on the task's type:

  - For tasklets: we use tasklet_wakeup()
  - For tasks: we use task_wakeup()

If 68e692da0 ("MINOR: event_hdl: add event handler base api")
is being backported, then this commit should be backported with it.