system: add option to install additional init scripts
system/skeleton: fixup /etc/fstab
board/ryco/configs: add minimal read-only config
board/ryco: add partition 2 to raspberry pi fstab file
system: add option to load specific kernel modules during startup
board/ryco/configs: update configs
skeleton: update rcS
sdl2: new package
sdl2_gfx: new package
sdl2_image: new package
sdl2_mixer: new package
sdl2_net: new package
sdl2_ttf: new package
mpd: bump to version 0.19.4

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
board/ryco: sync raspi-kernel.patch with upstream
rpi-firmware: bump version
board/ryco: update raspberry pi patchset