cbfstool: improve specification of struct cbfs_file

Lock down its size and document some of the fields

Change-Id: I09fd6c80185345da0ae17d0f4498b50995fd1ec5
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/10927
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

util/cbfstool/cbfs.h

@@ -71,12 +71,16 @@ struct cbfs_header {
 
 struct cbfs_file {
 	uint8_t magic[8];
+	/* length of file data */
 	uint32_t len;
 	uint32_t type;
 	uint32_t checksum;
+	/* length of header incl. variable data */
 	uint32_t offset;
 } __PACKED;
 
+_Static_assert(sizeof(struct cbfs_file) == 24, "cbfs_file size mismatch");
+
 struct cbfs_stage {
 	uint32_t compression;
 	uint64_t entry;