Loader: split add_only into own function

[Megamouse]

• Splits add_only code into own neat function that doesn't affect emulation
• Fixes missing games from "/games" when refreshing the game list during gameplay

I think it's better to have the boot and add stuff seperated, as it got really confusing over time and I realized there's not really much to it.
The new code does not require any modifications to the emulation state apart from loading and mounting parts of the VFS.
This also means less logging which confuses our discord bot.

I first copy pasted the loader code into a different file and started removing all the sections that are not affected by add_only.
Then I removed all the sections that were unreachable (unbeknownst to us).
And finally I reduced the remaining logic to simply log some notes instead of doing more analysis.

I'm sure some of the remaining code can be omited as well, but I am not confident to say what it's all about.