Allow specifying a root path for host0:/.

This way, similar to the PSP, we can have a root path outside the test.

UI/EmuScreen.cpp

@@ -89,6 +89,7 @@ void EmuScreen::bootGame(const std::string &filename) {
 	coreParam.enableSound = g_Config.bEnableSound;
 	coreParam.fileToStart = filename;
 	coreParam.mountIso = "";
+	coreParam.mountRoot = "";
 	coreParam.startPaused = false;
 	coreParam.printfEmuLog = false;
 	coreParam.headLess = false;