buildroot

configs: snps_archs38_vdk: use same headers as kernel

Instead of downloading a *different* version of the kernel for the
headers as for the headers itself, use the same one.
BR2_KERNEL_HEADERS_AS_KERNEL is the default, but we have to set the
custom version.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>