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).
What do you think about renaming this to "use_virtual_sd"? Calling it SDMC requires knowledge of the 3DS system to actually grasp what this option is doing, which I'd like to avoid (let's not end in a situation where each developer only understands half of the emulator features).