diffoscope incompatible with coreboot 4.21 cbfs

The tests from test_cbfs.py fail with coreboot 4.21.

The size of the cbfs_header changend and cbfs.py no longer matches it. I suspect this is a coreboot bug, but even if it is, it may be useful for diffoscope to be able to recognize files produced by coreboot 4.21.

More detail in https://github.com/NixOS/nixpkgs/issues/256896