[Merge] ~xypron/livecd-rootfs:riscv-xubuntu into livecd-rootfs:ubuntu/master

[Michael Hudson-Doyle]

> $SUBARCH is already checked in live-build/auto/config before
> export IMAGE_TARGETS="disk-image-non-cloud"

Ah, I missed that detail. I've merged this now (but not uploaded it yet)
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https://code.launchpad.net/~xypron/livecd-rootfs/+git/livecd-rootfs/+merge/494238
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