Gate use of custom directories behind a variable (#6157)

previous changes had forced every single user to use custom
directories for NAND and SDMC. Those paths were saved to the
config file and would interact badly with portable builds.

src/citra/default_ini.h

@@ -250,6 +250,10 @@ volume =
 # 1 (default): Yes, 0: No
 use_virtual_sd =
 
+# Whether to use custom storage locations
+# 1: Yes, 0 (default): No
+use_custom_storage =
+
 # The path of the virtual SD card directory.
 # empty (default) will use the user_path
 sdmc_directory =