build system: don't try to add cbfs-files with no backing file

Change-Id: Idd05a552762be92d0d93b357b96442b25a614757
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/12537
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
1 file changed