[Merge] ~xnox/livecd-rootfs:disk-images-all-the-things into livecd-rootfs:ubuntu/master

[Dimitri John Ledkov]

Dimitri John Ledkov has proposed merging ~xnox/livecd-rootfs:disk-images-all-the-things into livecd-rootfs:ubuntu/master.

Commit message:
disk-images all the things.

1) fix skipping building MBR images
2) explicitely build amd64 MBR image
3) skip MBR image on riscv64
4) add UEFI image on riscv64 without a bootloader
5) add secureboot support to arm64 image

This is draft/work in progress, need to upload this and test all of these builds first for correctness.

Individual commits are self contained and have informative messages as to what is being changed for audit purposes.

Requested reviews:
  Canonical Cloudware (cloudware)
  Ubuntu Core Development Team (ubuntu-core-dev)

For more details, see:
https://code.launchpad.net/~xnox/livecd-rootfs/+git/livecd-rootfs/+merge/394034
-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~xnox/livecd-rootfs:disk-images-all-the-things into livecd-rootfs:ubuntu/master.
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