[3DS] Provide autorun functionality for forwarders and custom frontends

It would be useful to have autorun functionality on the 3DS version of RetroArch. Other systems have this with command lines, but this isn't an option with 3DS homebrew development.

On the 3DS, the community has already developed "forwarders", which allow users to create items on the Home Menu to launch specific roms in RetroArch. The issue here, is it requires a custom copy of RA for every rom baked into the forwarder. This seems highly inefficient, and there is no way for the users to update the version of RetroArch these forwarders use.

What would be useful would be for 3DS RetroArch to detect for the existence of a autorun ini or cfg file. And if this file exists, read the file path from it. The file would be created by an external app (either a forwarder or custom frontend). Once read, delete the file, and autoload the rom at the target path if its compatible with the core in "retroarch-salamander.cfg".

This would bring autorun functionality to the 3DS, meaning more efficient forwarders could be built by the homebrew community, which would utilise a centralised and user updatable copy of RetroArch.

Start by locating RetroArch's 3DS startup and launch handling, then trace how retroarch-salamander.cfg is read and how core compatibility is checked. Define the autorun file format and its location from the 3DS environment, including when it is deleted. Done means a compatible ROM can be launched through the autorun file and incompatible targets are not loaded.