kernel: bump to 3.12.28

firmware: Add config option hdmi_blanking to allow hdmi output to be disabled when framebuffer is blanked
See: https://github.com/popcornmix/omxplayer/pull/247

firmware: Keep built files out of source tree to avoid tainted builds
See: https://github.com/Hexxeh/rpi-update/issues/146
kernel: bcm2708: Make the new MMC be enabled by default

See: http://www.raspberrypi.org/forums/viewtopic.php?f=63&t=85061
kernel: Some improvements for I2S audio
See: https://github.com/raspberrypi/linux/issues/681

kernel: bcm2708_fb: Reduce dmesg spam
See: https://github.com/raspberrypi/linux/issues/685

kernel: Added driver for HiFiBerry Amp amplifier add-on board
See: https://github.com/raspberrypi/linux/pull/659

MMAL: Rebuilt with updated MMAL makefiles
Add missing files from last update
kernel: bcm2708_i2s avoid address space leak
See: https://github.com/raspberrypi/linux/issues/687

firmware: hdmi: Add gencmd for controlling small adjustments to hdmi pixel clock
kernel: upgrade to 3.16.3
firmware: merge upstream