mkosi: Package a erofs usr partition with signed verity

mkosi: Package a erofs usr partition with signed verity

Let's start moving towards a more involved partitioning setup to
test our stuff more when using mkosi.

The root partition is generated on boot with systemd-repart.

CentOS supports neither erofs nor btrfs so we use squashfs and xfs
instead.

We also enable SecureBoot= locally for additional coverage. This
and the use of verity means users need to run `mkosi genkey` once
to generate the keys necessary to do secure boot and verity.