kernel: Bump to 3.12.21

kernel: config: Add CONFIG_IPV6_PRIVACY
See: https://github.com/raspberrypi/linux/issues/612

kernel: vchiq: Include SIGTRAP in list of signals not-masked by vchiq to allow gdb to work
See: http://www.raspberrypi.org/forums/viewtopic.php?f=67&t=76377

firmware: deinterlace: Improve 1080i deinterlace
Also deinterlaces chroma. Bilinear interpolate duplicated pixels