[Merge] ~vorlon/livecd-rootfs:subarch-manifest into livecd-rootfs:ubuntu/master

[Heinrich Schuchardt]

Building our preinstalled RISC-V QEMU images (riscv64:generic) would fail with this patch.

For which ARCH, SUBARCH we can build depends on PROJECT, SUBPROJECT.
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https://code.launchpad.net/~vorlon/livecd-rootfs/+git/livecd-rootfs/+merge/451944
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