Makefile: Give a build error if ad-hoc CONFIG options are added

New CONFIG options should be added via Kconfig. To help prevent new ad-hoc
CONFIGs from being added, give a build error when these are detected.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>