kconfig: fix file name in warnings when loading KCONFIG_DEFCONFIG_LIST

Most 'make *config' commands use .config as the base configuration file.

When .config does not exist, Kconfig tries to load a file listed in
KCONFIG_DEFCONFIG_LIST instead.

However, since commit b75b0a81 ("kconfig: change defconfig_list
option to environment variable"), warning messages have displayed an
incorrect file name in such cases.

Below is a demonstration using Debian Trixie. While loading
/boot/config-6.12.9-amd64, the warning messages incorrectly show .config
as the file name.

With this commit, the correct file name is displayed in warnings.

[Before]

  $ rm -f .config
  $ make config
  #
  # using defaults found in /boot/config-6.12.9-amd64
  #
  .config:6804:warning: symbol value 'm' invalid for FB_BACKLIGHT
  .config:9895:warning: symbol value 'm' invalid for ANDROID_BINDER_IPC

[After]

  $ rm -f .config
  $ make config
  #
  # using defaults found in /boot/config-6.12.9-amd64
  #
  /boot/config-6.12.9-amd64:6804:warning: symbol value 'm' invalid for FB_BACKLIGHT
  /boot/config-6.12.9-amd64:9895:warning: symbol value 'm' invalid for ANDROID_BINDER_IPC

Fixes: b75b0a81 ("kconfig: change defconfig_list option to environment variable")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>