linux-wandboard (3.10.17): Update SRCREV to 4a8ad4

Update Wandboard 3.10.17 kernel to commit 4a8ad4, which includes an update
to the DTS to enable all mxcfb devices and a fix for the sgtl5000 driver.

Signed-off-by: John Weber <rjohnweber@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
wandboard: Change kernel to 3.10.17

Move default kernel selection to 3.10.17 in advance of the 1.6 release,
because currently 3.0.35_4.0.0 is broken in master.  There are known issues
with the kernel that will be resolved with community assistance.  These
include:

- HDMI:  Using X11 causes HDMI output to default to 1024x768 resolution
         regardless of the 'video=mxcfb...' bootarg selection.
- HDMI, EDID:  Does not work (but this did not work on 3.0.35 either).
- Sound, HDMI audio output: Using HDMI audio output seems to cause very slow
         playback of both audio and video, and can be seen using aplay or
         gst-launch.

Signed-off-by: John Weber <rjohnweber@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>