[Feature Request] Expose e-Reader via disc control interface

[Sanaki]

This issue is being filed mostly in the hopes that we can start easily making achievement sets over on RetroAchievements. Having tested out the e-Reader functionality in standalone, it seems fully functional and compatible with this concept. Essentially it'd work as follows:

• The e-Reader hardware dump (No-Intro name e-Reader (USA).gba) is stored in the system directory as a "bios". (ereader.bin?)
• Loading an e-Reader dotcode dump (bin/raw/bmp) or an m3u file referencing them will boot the e-Reader "bios" with the selected file loaded and ready to scan. Further dotcode can be loaded as needed via the disc control interface, though m3u is likely preferable for ease of use.

Since the dotcode dump is the file being loaded, this allows saving the loaded program on a per-card basis rather than having to keep only one saved. Similarly, this allows us at RA to hash the dotcode for individual achievement sets rather than having to jam -every- minigame under a single entry.

Technically e-Reader games are playable now on the libretro core, but only via manually porting a save file from the standalone, and only one can be available at a time without duplicating the e-Reader hardware dump for each desired game.