build-kernel: Use make target to process defconfig

This allows us to use trimmed defconfigs which are way easier to
maintain and compare.

build/build_kernel_deb.sh

@@ -112,7 +112,7 @@ if [ ! -e .config ]; then
   if [ -e "../${DOT_CONFIG}" ]; then
     cp "../${DOT_CONFIG}" .config
   else
-    cp "arch/${ARCH}/configs/${DOT_CONFIG}" .config
+    make "${MAKE_OPTS[@]}" ARCH="${ARCH}" CROSS_COMPILE="${CROSS_COMPILER}" "${DOT_CONFIG}"
   fi
 fi