scripts: ci: check_compliance: add per-file Kconfig allowlist

Add scripts/ci/undef_kconfig_files_allowlist.txt, a new configuration
file that lists source files to skip entirely during the undefined
Kconfig symbol check (check_no_undef_outside_kconfig). All CONFIG_
references found in a listed file are ignored, avoiding the need to
add each symbol individually to undef_kconfig_allowlist.txt when an
entire file is known to be a false-positive source (e.g. generated
files or documentation referencing out-of-tree symbols).

The file list is loaded from the default path at run time, and can be
extended via the UNDEF_KCONFIG_FILES_ALLOWLIST_FILE environment
variable, consistent with the existing UNDEF_KCONFIG_OUTSIDE_ALLOWLIST_FILE
mechanism.

Assisted-by: Claude:claude-sonnet-4.6
Signed-off-by: Anas Nashif <anas.nashif@intel.com>