[PATCH v5] 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.
Handle properly if the config file is a relative path.

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

How the patch was tested:
	make mrproper
	export KBUILD_OUTPUT=.out
	make defconfig
	grep "CONFIG_UBSAN=y" $KBUILD_OUTPUT/.config || echo "OK"

Test the basic usage:
	./scripts/config -e CONFIG_UBSAN
	grep "CONFIG_UBSAN=y" $KBUILD_OUTPUT/.config && echo "OK"

Test --refresh picks up correct config file by using $KBUILD_OUTPUT:
	./scripts/config -d CONFIG_UBSAN
	./scripts/config --refresh -e CONFIG_UBSAN
	grep "CONFIG_UBSAN=y" $KBUILD_OUTPUT/.config && echo "OK"

Test --file takes precedence over $KBUILD_OUTPUT and a relative
path is handled properly:
	./scripts/config -d CONFIG_UBSAN
	mkdir .out2
	cp .out/.config .out2/.my_config
	grep "CONFIG_UBSAN=y" .out2/.my_config
	./scripts/config --file .out2/.my_config --refresh \
	       -e CONFIG_UBSAN
	grep "CONFIG_UBSAN=y" .out2/.my_config && echo "OK"
	ls .out/.out2 || echo "OK"
	ls .out/.out || echo "OK"

Test --file can accept files with hyphen:
	./scripts/config -d CONFIG_UBSAN
	cp .out/.config -- -my_config
	grep "CONFIG_UBSAN=y" -- -my_config || echo "OK"
	./scripts/config  --refresh \
	       -e CONFIG_UBSAN --file -my_config
	grep "CONFIG_UBSAN=y" -- -my_config && echo "OK"

Signed-off-by: Dmitry Voytik <voytikd@gmail.com>
---
v5:
 - Per Nathan Chancellor's suggestion, improved handling $FN path by
   realpath
 - https://lore.kernel.org/lkml/179097210878.309835.10523307262544578337.b4-review@b4/
 - Documented the new test scenario in the git commit message
v4:
 - Addressed Nicolas Schier's comments - dropped the change how '--refresh'
   is handled
 - Implemented conversion of $FN to the absolute path which helps with
   '--refresh' and relative paths
 - Documented testing steps in the commit message
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 | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/scripts/config b/scripts/config
index 32428ea909c2..7e84c3d4e621 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
@@ -143,6 +147,7 @@ while [[ $# -gt 0 ]]; do
 		shift
 	fi
 done
+FN=$(realpath -q -- "$FN" || echo "$FN")
 
 set -- "${CMDS[@]}"
 if [ "$1" = "" ] ; then
-- 
2.55.0