kernel: Don't check kernel config for debug kernels

The kernel config for debug kernels is created by concatenating
config files, so we can't use diff to check it.

This fixes a regression introduced by:
9362de0a ("kernel: Verify kernel config")

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

kernel/Dockerfile

@@ -86,7 +86,7 @@ RUN case $(uname -m) in \
     rm /linux/kernel_config-${KERNEL_SERIES}* && \
     make defconfig && \
     make oldconfig && \
-    diff .config ${KERNEL_DEF_CONF}
+    if [ -z "${DEBUG}" ]; then diff .config ${KERNEL_DEF_CONF}; fi
 
 RUN mkdir /out