fix git dir detection in worktrees

scripts/configure-kernel.sh

@@ -91,7 +91,9 @@ function modify_kernel_config() {
 function configure_kernel() {
     if [[ "${CLEAN_BUILD}" == '1' ]]; then
         rm -vf .config*
-        [[ -d .git ]] && git clean -x -d -f
+        if [[ "$(git rev-parse --is-inside-work-tree)" == 'true' ]]; then
+            git clean -x -d -f
+        fi
         $SUDO_ALIAS make distclean
     fi