Add signed kernel check to prevent pushing improperly signed kernels

[bsherman]

We shold add a step after image build and before push which confirms kernel in the image was signed with the proper key.

Can use: podman run --entrypoint /bin/bash image -c "cat path to vmlinuz" > tmp/vmlinuz

And then do a sbverify --cert

[m2Giles]

In downstream images this has been implemented.

But we can do something similar in this repo.

[m2Giles]

Implemented in #18