[PATCH v3] scripts/config: honor $KBUILD_OUTPUT by default

[Dmitry Voytik <voytikd@gmail.com>]

If $KBUILD_OUTPUT is set, use it by default if --file is not passed.

Fix the issue when the script silently updates a stale .config in
the root directory, while the user expects ${KBUILD_OUTPUT}/.config
to be updated.
The problem can be worked around with --file, but this is easy to miss,
and is inconsistent with how the Makefile and other tools work
(e.g., scripts/diffconfig).

How to reproduce the issue:
	rm .config
	export KBUILD_OUTPUT=.out
	make defconfig
	./scripts/config \
	       -e CONFIG_UBSAN
	grep: .config: No such file or directory

Signed-off-by: Dmitry Voytik <voytikd@gmail.com>
---
v3:
 - Added an example in the commit message how to reproduce the issue
v2:
 - Addressed comments by sashiko:
   - https://sashiko.dev/#/patchset/20260920064350.17999-1-voytikd%40gmail.com
   - use basename when --refresh is used
   - remove echo which can be misleading when --file is passed
v1:
 - https://lore.kernel.org/all/20260920064350.17999-1-voytikd@gmail.com/
---
 scripts/config | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/scripts/config b/scripts/config
index 32428ea909c2..cdee67157fe1 100755
--- a/scripts/config
+++ b/scripts/config
@@ -130,6 +130,10 @@ on_exit() {
 trap on_exit EXIT
 
 FN=.config
+if [ -n "${KBUILD_OUTPUT}" ]; then
+	FN=${KBUILD_OUTPUT}/${FN}
+fi
+
 CMDS=()
 while [[ $# -gt 0 ]]; do
 	if [ "$1" = "--file" ]; then
@@ -229,7 +233,7 @@ while [ "$1" != "" ] ; do
 		;;
 
 	--refresh)
-		yes "" | make oldconfig KCONFIG_CONFIG=$FN
+		yes "" | make oldconfig KCONFIG_CONFIG=$(basename $FN)
 		;;
 
 	*)
-- 
2.55.0