Use config files to store whether SDMC is enabled or not, auto-create SDMC dir.

Before, it used to use whether the directory actually existed. As a result, .citra-emu/sdmc was never auto-created (something quite confusing to me until I read through the logs).
I’m planning on having a nand directory for savegames, alongside the sdmc one, exposing the native nand hierarchy: http://3dbrew.org/wiki/Flash_Filesystem