buildroot: use QEMU overlay only when platform is QEMU or QEMUv8

common.mk mistakenly uses the QEMU overlay (which currently contains
only an init script for udhcpc) on all platforms. This is incorrect and
causes problems on RPi3 at least [1].

Fixes: commit 751b35bb ("qemu: buildroot: fix networking")
Links: [1] https://github.com/OP-TEE/optee_os/issues/2478#issuecomment-432818199


Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>