After '6392a63 rpi-base.inc: Use KERNEL_DEVICETREE by default' was
introduced, kernel versions < 3.18 might not be buildable. Since full
device tree support was introduced in 3.18 this change ensures that all
kernel < 3.18 will automatically disable device tree.
Signed-off-by: Petter Mabäcker <petter@technux.se>
