FEXQonfig: Allow system-provided RootFS images

[WhatAmISupposedToPutHere]

Add a secondary RootFS directory that can be managed by the system package manager.

[alyssarosenzweig]

Could you explain a bit on the background for this?

For Asahi specifically, the system rootfs will end up living in /run/share/fex-emu, I think, due to krun magic.

[WhatAmISupposedToPutHere]

For gentoo, i do not want to do krun magic inside the vm, and instead create the squashfs and overlay mounts on the host. Since mounting it inside the per-user rootfs directory would be quite annoying to implement, I instead want to add a secondary "default" rootfs search directory.

[WhatAmISupposedToPutHere]

And while the current gentoo work is asahi-specific, i want to also run it on my rk3588 boards, which will not use krun.

[alyssarosenzweig]

OK, makes sense. Not too different from the fedora asahi use case then..

[Sonicadvance1]

I had a couple of concerns that I didn't have time to articulate before today. So I spent a bit of time to implement this closer to what FEX would want.

• Hardcoded global paths, we would want FEX to follow what was provided with CMAKE_INSTALL_PREFIX
  • Likely being /usr or /usr/local typically, depending on what the user wants.
• We want FEXCore to track the global data path so it can tie in to our configuration system, even if only used for this rootfs implementation today.
• There was some extraneous prefixing thing in this PR which wasn't very necessary, we can just show the full system path instead.
  • This was likely a confusion for how FEX's "named" rootfs behaviour works, which isn't necessary for this global path, just supply the full path.
• I wanted to reimplement the inotify interface to use ppoll instead of select.
  • This was just my bad implementation from the start
• The watch descriptors didn't need to be saved in the global object so I wanted to change that (We don't support removing a WD from an inotify on full path deletion)

Thank you for bringing up that you wanted this as a feature in FEXConfig!

[Sonicadvance1]

Handled by #4074. If there's any more features you want then make sure to let us know.