kernel: bump to 3.10.21

kernel: config: Add CONFIG_IFB
See: https://github.com/raspberrypi/linux/issues/453
See: https://github.com/raspberrypi/linux/issues/445

kernel: dvb: Add support for CableStar Device
See: https://github.com/raspberrypi/linux/pull/449

kernel: sdhci: Only do one iteration of PIO reading loop (from gsh)