Use config files to store whether SDMC is enabled or not

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).
Common: Return from CreateFullPath early if the directory creation fails
Merge pull request #133 from archshift/sdmc-enabled

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