kernel: bump to 3.12.21

kernel: config: Add CONFIG_HID_XINMO
See: https://github.com/raspberrypi/linux/issues/609

firmware: audio_render/audio_mixer: Support planar formats

firmware: bootcode: increase sdcard timeout to avoid observed boot failure under certain conditions

firmware: deinterlace_double - fix image locking
See: https://github.com/raspberrypi/firmware/issues/181

firmware: Convert image_fx for use with MMAL and MMAL_ENCODING_OPAQUE
See: https://github.com/raspberrypi/firmware/issues/278

firmware: RIL Camera: fix handling for stride>required stride
See: https://github.com/raspberrypi/userland/issues/177