Code review comment for lp://staging/~ltrager/curtin/lp1640519

> I ended up running into another bug while testing. Curtin updates the initrd a
> second time on ARM but wasn't mounting /dev which caused update-initramfs to
> fail. Instead of calling util.subp update_initramfs now uses
> util.ChrootableTarget which automatically mounts /dev, /sys, and /proc. After
> making that change I was able to deploy Xenial on an ARM host.

Nice catch! That warrants another unittest in the commands curthooks.