In browser on oled, add '<' for readonly files

Also:
- fix potential out by error/buffer overflow
- code formatting cleanup
- remove superfluous *.d64 filename check

src/FileBrowser.cpp

@@ -528,7 +528,7 @@ void FileBrowser::FolderChanged()
 
 void FileBrowser::DisplayRoot()
 {
-	f_chdir("\\1541");
+	f_chdir("/1541");
 	FolderChanged();
 }