add stealth autoboot

[jmlee337]

screen stays black until the game shows up (unless there's a loader error). activated only when

• autoboot is turned on
• enable_stealth_autoboot.txt (with any contents) is present in the same dir as slippi_nincfg.bin

thus impossible to turn on accidentally
also acts as a settings lock

[jmlee337]

#56

[jmlee337]

I've discovered an issue where a wii that has problems (won't take inputs) but can boot on 1.13.0 doesn't work at all on this patch (exits to loader)