check_whence: Recognize RawFile keyword

WHENCE and copy_firmware.sh now support a new keyword RawFile to list
files that must not be compressed. Update check_whence.py to recognize
that.

Signed-off-by: Juerg Haefliger <juerg.haefliger@canonical.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>