build: Fix config file warning message
build: Fix config file warning message

Commit ffe6ccfc9047245c0d6193738b00973f2ee5460c inadvertently removed the
`CONFIG_IN` variable used by the build to generate part of the warning
message in the auto-generated config file.

Re-added to fix it.

Fixes #635.

Signed-off-by: James O. D. Hunt <348fbaadfbada55ea105602bf5609e23ac1c4898@intel.com>

clearcontainers/runtime,devimc/runtime,clearcontainers/runtime,chavafg/runtime,chavafg/runtime,devimc/runtime