Log Message:
emulators: Add libretro-smsplus

Libretro is a simple but powerful development interface that allows for the
easy creation of emulators, games and multimedia applications that can plug
straight into any libretro-compatible frontend. This development interface is
open to others so that they can run these pluggable emulator and game cores
also in their own programs or devices.

SMS Plus is an open source Sega Master System and Game Gear emulator written
by Charles MacDonald.

SMS Plus GX is an enhanced version which includes improved accuracy and bug
fixes.