Polish shutdown experience

[amstan]

Because the rootfs is more delicate than Mister's OS we should probably look at making shutting down easier.

We have 2 problems to solve: a way to shutdown, notification for it finishing.

A way to shutdown the system from the UI. I guess a script that calls systemctl poweroff might be enough.

Some feedback for the user that the shutdown finished. I declare that It must look like the old windows one, same font, but perhaps the text changed to s/computer/MiSTer to not confuse it with an AO486 core running windows 98.

The shutdown message needs to be implemented in some kind of single purpose core (I noticed ao486 core is happly executing and display even after linux was powered off). I could imagine Mister upstream might like such a core too. Or perhaps the linux framebuffer could be used instead?

In terms of triggering, systemd can probably easily be made to load a one shot service which loads a new rbf just before shutdown (after filesystem was unmounted).

[amstan]

systemctl poweroff

Hmm, this does shutdown, but eventually we get rebooted.

I guess we neeed to adjust systemd's poweroff method to just hang after filesystem unmounting instead of calling the kernel stuff (this will also keep the framebuffer active).

For the message we could fbi shutdown.png --noverbose -1 from fbida. But we still need to somehow make sure we're running the menu core and it's set to F9 mode. https://misterfpga.org/viewtopic.php?f=27&t=524 might be related.