Add support for logging

[yifanlu]

Here's an idea: enable logging support by checking if "sdmc:/loader.txt" exists. If it does not, do not enable logging (save resources). If it does exist, open that file with append permissions. Set the console to write to the log and you can use printf within the code. Would be very useful for debugging. (Just create an empty file named loader.txt and when you're done, delete it and logging is disabled).

I didn't do this because this would make loader way too big. However, if you're willing to rebuild FIRM, size doesn't matter.