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/Boot/Boot.h

@@ -5,15 +5,13 @@
 #pragma once
 
 #include <cstdlib>
+#include <optional>
 #include <string>
+#include <variant>
 
 #include "Common/CommonTypes.h"
-
-namespace DiscIO
-{
-class Volume;
-struct Partition;
-}
+#include "DiscIO/Enums.h"
+#include "DiscIO/Volume.h"
 
 struct RegionSetting
 {
@@ -23,10 +21,57 @@ struct RegionSetting
   const std::string code;
 };
 
+struct BootParameters
+{
+  struct Disc
+  {
+    std::string path;
+    std::unique_ptr<DiscIO::Volume> volume;
+  };
+
+  struct Executable
+  {
+    enum class Type
+    {
+      DOL,
+      ELF,
+    };
+    std::string path;
+    Type type;
+  };
+
+  struct NAND
+  {
+    std::string content_path;
+  };
+
+  struct IPL
+  {
+    explicit IPL(DiscIO::Region region_);
+    IPL(DiscIO::Region region_, Disc&& disc_);
+    std::string path;
+    DiscIO::Region region;
+    // It is possible to boot the IPL with a disc inserted (with "skip IPL" disabled).
+    std::optional<Disc> disc;
+  };
+
+  struct DFF
+  {
+    std::string dff_path;
+  };
+
+  static std::unique_ptr<BootParameters> GenerateFromFile(const std::string& path);
+
+  using Parameters = std::variant<Disc, Executable, NAND, IPL, DFF>;
+  BootParameters(Parameters&& parameters_);
+
+  Parameters parameters;
+};
+
 class CBoot
 {
 public:
-  static bool BootUp();
+  static bool BootUp(std::unique_ptr<BootParameters> boot);
   static bool IsElfWii(const std::string& filename);
 
   // Tries to find a map file for the current game by looking first in the