Boot: Clean up the boot code

* 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.

Source/Core/Core/Core.h

@@ -11,11 +11,14 @@
 #pragma once
 
 #include <functional>
+#include <memory>
 #include <string>
 #include <vector>
 
 #include "Common/CommonTypes.h"
 
+struct BootParameters;
+
 namespace Core
 {
 bool GetIsThrottlerTempDisabled();
@@ -31,7 +34,7 @@ enum class State
   Stopping
 };
 
-bool Init();
+bool Init(std::unique_ptr<BootParameters> boot);
 void Stop();
 void Shutdown();