Add SecureBoot support for arbitrary distributions

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I outlined an idea that would actually make distribution of SecureBoot-signed bootloader files much easier:

RFC: Distribution of netboot files via OCI registry

Hello there, it’s been a while since I started a thread, but here am I, cheers! Recently, I was playing around with ORAS tool and an idea came to my mind. Problem: in order to prepare PXE environment on TFTP/HTTP smart proxy, Foreman downloads installer image (kernel + init RAM disk) from linux distribution repository when a host is created. Then a bootloader, which is installed by foreman-installer, is used to load installer into memory. There are couple of problems with this approach: If t…