sdl2: new package
sdl_gfx: allow building with sdl or sdl2
sdl_image: allow building with sdl or sdl2
sdl_mixer: allow building with sdl or sdl2
sdl_net: allow building with sdl or sdl2
sdl_ttf: allow building with sdl or sdl2
emulationstation: bump version

replace two sed commands with one
add hack to main.cpp to set environment for sdl2
board/ryco/configs: rename rpi_emulationstation.config to rpi_retroarch.config
rpi-userland: rename patches to new convention
emulationstation: update guimenu patch
board/ryco/overlay/retroarch: remove obsolete modules.conf
skeleton: update rcS
board/ryco/overlay/retroarch: use startup script to handle retroarch and emulationstation