Add a mode to use a dummy Wii NAND.

Eventually, netplay will be able to use the host's NAND, but this could
still be useful in some cases; for TAS it definitely makes sense to have
a way to avoid using any preexisting NAND.

In terms of implementation: remove D_WIIUSER_IDX, which was just WIIROOT
+ "/", as well as some other indices which are pointless to have as
separate variables rather than just using the actual path (fixed, since
they're actual Wii NAND paths) at the call site.  Then split off
D_SESSION_WIIROOT_IDX, which can point to the dummy NAND directory, from
D_WIIROOT_IDX, which always points to the "real" one the user
configured.

Source/Core/Common/SysConf.cpp

@@ -8,6 +8,7 @@
 #include <string>
 #include <vector>
 
+#include "Common/CommonPaths.h"
 #include "Common/CommonTypes.h"
 #include "Common/FileUtil.h"
 #include "Common/SysConf.h"
@@ -15,8 +16,7 @@
 SysConf::SysConf()
 	: m_IsValid(false)
 {
-	m_FilenameDefault = File::GetUserPath(F_WIISYSCONF_IDX);
-	m_IsValid = LoadFromFile(m_FilenameDefault);
+	UpdateLocation();
 }
 
 SysConf::~SysConf()
@@ -409,7 +409,10 @@ void SysConf::UpdateLocation()
 	// Clear the old filename and set the default filename to the new user path
 	// So that it can be generated if the file does not exist in the new location
 	m_Filename.clear();
-	m_FilenameDefault =  File::GetUserPath(F_WIISYSCONF_IDX);
+	// Note: We don't use the dummy Wii root here (if in use) because this is
+	// all tied up with the configuration code.  In the future this should
+	// probably just be synced with the other settings.
+	m_FilenameDefault = File::GetUserPath(D_WIIROOT_IDX) + DIR_SEP WII_SYSCONF_DIR DIR_SEP WII_SYSCONF;
 	Reload();
 }