fix(mtk-vendor): drop VERSION_CODE_FILENAMES for clean image basenames

The fork's image-config.in defaults CONFIG_VERSION_CODE_FILENAMES to y,
which prefixes images with the revision token (e.g. -r0-a828161-).
Upstream ImmortalWRT doesn't ship that, so firmware names diverged from
every other BananaWRT channel and old update scripts built from
FIRMWARE_VERSION alone couldn't reconstruct the path. Disable the knob
to keep 'immortalwrt-25.12-linkup-mediatek-filogic-...' as the final
shape.

Author: SuperKali

config/v25.12-mtk-vendor/mtk-vendor/.config

@@ -451,6 +451,7 @@ CONFIG_PACKAGE_libblkid=y
 CONFIG_IMAGEOPT=y
 CONFIG_VERSIONOPT=y
 CONFIG_VERSION_CODE=""
+# CONFIG_VERSION_CODE_FILENAMES is not set
 CONFIG_VERSION_DIST="ImmortalWrt"
 CONFIG_VERSION_FILENAMES=y
 CONFIG_VERSION_MANUFACTURER_URL=""