cmake: do not error on missing 'config'

'conf' is part of the zephyr SDK or need to be built and installed in
the path. We now using python for Kconfig processing, so this is not a
strict requirement and we should be able to build without it. If the
binary is not found, just go on with our business.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>