linux: update to linux-3.15.5
libXi: update to libXi-1.7.3

Signed-off-by: Stephan Raue <stephan@openelec.tv>
e2fsprogs: update to e2fsprogs-1.42.11

Signed-off-by: Stephan Raue <stephan@openelec.tv>
xbmc: build encoder addons only if optical drive support is enabled