* Move out boot parameters to a separate struct, which is not part
of SConfig/ConfigManager because there is no reason for it to
be there.
* Move out file name parsing and constructing the appropriate params
from paths to a separate function that does that, and only that.
* For every different boot type we support, add a proper struct with
only the required parameters, with descriptive names and use
std::variant to only store what we need.
* Clean up the bHLE_BS2 stuff which made no sense sometimes. Now
instead of using bHLE_BS2 for two different things, both for storing
the user config setting and as a runtime boot parameter,
we simply replace the Disc boot params with BootParameters::IPL.
* Const correctness so it's clear what can or cannot update the config.
* Drop unused parameters and unneeded checks.
* Make a few checks a lot more concise. (Looking at you, extension
checks for disc images.)
* Remove a mildly terrible workaround where we needed to pass an empty
string in order to boot the GC IPL without any game inserted.
(Not required anymore thanks to std::variant and std::optional.)
The motivation for this are multiple: cleaning up and being able to add
support for booting an installed NAND title. Without this change, it'd
be pretty much impossible to implement that.
Also, using std::visit with std::variant makes the compiler do
additional type checks: now we're guaranteed that the boot code will
handle all boot types and no invalid boot type will be possible.
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