kernel: Bump to 3.12.20

kernel: V4L2 padding removal, and support for H264_I_PERIOD
See: https://github.com/raspberrypi/linux/pull/598

firmware: dispmanx: Limit LBM usage on any channel
See: https://github.com/raspberrypi/firmware/issues/277

firmware: MMAL: vc_image: Fix some of the combinations for vc_image_pack
firmware: MMAL: Don't ask for YUVUV when tearing down an opaque tunnel
See: http://www.raspberrypi.org/forums/viewtopic.php?f=43&t=62364&start=300#p552913

firmware: gencmd: Add display_power command for controlling power to hdmi phy

firmware: config: Add hdmi_force_mode option to force only one mode
See: http://openelec.tv/forum/124-raspberry-pi/71222-display-issues#108236

firmware: hdmi: Make configured mode have high score, so it is respected after power on preferred

firmware: hdmi: Call the callback with current hdmi mode, not preferred
See: http://forum.stmlabs.com/showthread.php?tid=11003&pid=102973#pid102973

firmware: dispmanx: Move the offline/rotated buffer allocation into attach function.
See: https://github.com/raspberrypi/linux/issues/463