buildroot

wandboard: Use kernel version 4.7.3

Wandboard is well supported in the mainline kernel, so let's switch
from the 3.14 vendor kernel to 4.7.3 mainline kernel version.

As mainline use more dts files to support both B1 and C1 versions of the
board, adjust the dts entries accordingly.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>