ffmpeg: change to official release

Signed-off-by: Stephan Raue <stephan@openelec.tv>
bcm2835-driver: update to bcm2835-driver-1e7d3d9

Signed-off-by: Stephan Raue <stephan@openelec.tv>
bcm2835-bootloader: update to bcm2835-bootloader-1e7d3d9

Signed-off-by: Stephan Raue <stephan@openelec.tv>
linux: update to linux-3.15.4

Signed-off-by: Stephan Raue <stephan@openelec.tv>