[Enhancement] e-Reader support for mGBA libretro core

[nijoakim]

I suggest porting the e-Reader functionality to libretro. It could make use RetroArch's subsystems functionality – The feature that enabled the emulation of the Nintendo 64 transfer pak.

A subtle benefit: E-reader functionality would be accessible in an emulator that supports RetroAchievements. Thus, achievements which require the e-Reader could be created.