When a target configuration has unser Kconfig symbols, the build will
fail when OpenWrt is compiled with V=s and stdin is connected to a tty.

In case OpenWrt is compiled without either of these preconditions, the
build will uscceed with the symbols in question being unset.

Modify the kernel configuration in a way it fails on unset symbols
regardless of the aformentioned preconditions.

Signed-off-by: David Bauer <mail@david-bauer.net>