Allow using dbxupdate.bin which contains the KEK

It looks like python-uefivars can only handle a dbxupdate siglist so the [AWS documentation](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/aws-binary-blob-creation.html) mentions under step 7) that the dbxupdate.bin file can be split into the binary data and the signature. But the links mentioned there are only useful for Windows powershell users. There are no instructions how to do that on Linux.
So it would be cool if python-uefivars could automatically split dbxupdate.bin and only use the siglist from it.