stdlib: Update AbstractCore's 'set_inst_stop_any_thread'

This patch:
- Makes this function private.
- Updates the function's documentation.
- Changes the 'init' parameter to 'board_initialized'.

It doesn't make much sense for this function to be exposed directly to
the user as it requires knowing whether the board is initialized or not.

In addition to this I believe it makes more sense for the 'init' logic
to be flipped and renamed "board_initialized' so that this value is True
if the board has been initialized.

The documentation for this function has been updated.

Change-Id: I016c65bde88357111d3e648d7aa99aeb6e31f410
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/64833
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

configs/example/gem5_library/checkpoints/simpoints-se-restore.py

@@ -132,5 +132,5 @@ simulator = Simulator(
 # is greater than 0.
 # In here, it schedules an exit event for the first SimPoint's warmup
 # instructions
-simulator.schedule_max_insts(board.get_simpoint().get_warmup_list()[0], True)
+simulator.schedule_max_insts(board.get_simpoint().get_warmup_list()[0])
 simulator.run()