Enable veth when using nbd boot option

Enable veth when using nbd boot option

This patch enables the VETH kernel config.
We use this to make sure the network binding finds a valid
ethernet device through connman even if 'eth0' is
used by nbd and thus not visible to connman.

Bug-AGL: SPEC-3631
Signed-off-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Change-Id: I056ff6dc8f83bdb707cfbac90562388c4e742b43
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25421
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>