buildroot

configs: snps_aarch64_vdk_defconfig: kernel pointing to 4.6

Previously the vdk solution was using a Linaro kernel that doesn't
build with gcc 5.x.

Since there is no point in using Linaro anymore for this solution,
this patch makes the aarch64 vdk defconfig point to the 4.6 Linux
kernel.

Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>