Bug#1053457: live-build: Building live image fails when unmounting /sys: 'target is busy'

[Emanuele Rocca <ema@debian.org>]

On 2023-10-04 03:10, Emanuele Rocca wrote:
>  P: Begin unmounting /sys...
>  umount: /tmp/sid-image/chroot/sys: target is busy.
>  E: An unexpected failure occurred, exiting...

The reason why /sys is busy is that efivarfs is mounted under
/sys/firmware/efi/efivars.

It seems that efivarfs gets mounted at the following stage during lb build:

 P: Begin installing packages (install pass)...
 [...]
 Nov 06 19:28:28 W: Possible missing firmware /lib/firmware/amdgpu/smu_13_0_10.bin for module amdgpu
 Nov 06 19:28:29 live-boot: core filesystems dm-verity devices utils udev blockdev dns.