cbfstool: rename checksum to attributes_offset

So far it's still unused, but its purpose will change:
It will become an offset to another structure that contains additional file
attributes.

This change is compatible because the binary format doesn't change and so far
the field was always set to 0, which can serve nicely as 'unused' field.

Change-Id: I2dafb06866713d43a236556f9492641526270837
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/10933
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
diff --git a/util/cbfstool/common.c b/util/cbfstool/common.c
index fffd096..d72db38 100644
--- a/util/cbfstool/common.c
+++ b/util/cbfstool/common.c
@@ -131,7 +131,7 @@
 	bgets(buf, &file->magic, sizeof(file->magic));
 	file->len = xdr_be.get32(buf);
 	file->type = xdr_be.get32(buf);
-	file->checksum = xdr_be.get32(buf);
+	file->attributes_offset = xdr_be.get32(buf);
 	file->offset = xdr_be.get32(buf);
 }