scripts: kconfig: merge_config.sh: fix unexpected operator warning

scripts: kconfig: merge_config.sh: fix unexpected operator warning

Fix a warning for:

$ ./scripts/kconfig/merge_config.sh .config extra.config
Using .config as base
Merging extra.config
./scripts/kconfig/merge_config.sh: 384: [: false: unexpected operator

The shellcheck report is also attached:

if [ "$STRICT" == "true" ] && [ "$STRICT_MODE_VIOLATED" == "true" ]; then
               ^-- SC3014 (warning): In POSIX sh, == in place of = is undefined.
                                                        ^-- SC3014 (warning): In POSIX sh, == in place of = is undefined.

Fixes: dfc97e1c5da5 ("scripts: kconfig: merge_config.sh: use awk in checks too")
Signed-off-by: Weizhao Ouyang <o451686892@gmail.com>
Reviewed-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Link: https://patch.msgid.link/20260309121505.40454-1-o451686892@gmail.com
Signed-off-by: Nathan Chancellor <nathan@kernel.org>