[Merge] livecd-rootfs:losetup-not-kpartx into livecd-rootfs:ubuntu/master

[Steve Langasek]

The topic has come up before of replacing kpartx with losetup.  kpartx seems to be racy, particularly on riscv64; we seem to get frequent build failures of the sort:

[...]
++ kpartx -s -v -a binary/boot/disk-uefi.ext4
/dev/mapper/loop2p15 not set up by udev: Falling back to direct node creation.
/dev/mapper/loop2p15: chown failed: No such file or directory
/dev/mapper/loop2p16 not set up by udev: Falling back to direct node creation.
/dev/mapper/loop2p16: chown failed: No such file or directory
[...]
+ create_and_mount_uefi_partition mountpoint
+ uefi_dev=/dev/mapper/loop2p15
+ mountpoint=mountpoint
+ mkfs.vfat -F 32 -n UEFI /dev/mapper/loop2p15
mkfs.vfat: unable to open /dev/mapper/loop2p15: No such file or directory
[...]

kpartx is dark and full of terrors.  Instead of debugging a race between libdevmapper and udev, we may be able to get better non-racy behavior from losetup.
-- 
https://code.launchpad.net/~ubuntu-core-dev/livecd-rootfs/+git/livecd-rootfs/+merge/441136
Your team Ubuntu Core Development Team is subscribed to branch livecd-rootfs:ubuntu/master.