hwdb: strip the root from filenames when generating hwdb.bin (#43062)

The modern hwdb.bin format contains the filenames of the input data that
makes up the database. This is useful but in offline builds where --root
is used, the filenames are the full build paths including the specified
root. This introduces build paths and thus information leakage and
non-reproducible data.

Solve this by stripping the root prefix off the original path when
passing to import_file.