fix(overlayfs): unmount NEWROOT before mounting overlay

fix(overlayfs): unmount NEWROOT before mounting overlay

When using `rd.overlay` the mount point `/run/rootfsbase` is not
accessible any more, because `LiveOS_rootfs` is mounted over it. This is
`/proc/mounts` from the `tmpfs overlay (rd.overlay)` test case:

```
tmpfs /run tmpfs rw,nosuid,nodev,size=194628k,nr_inodes=819200,mode=755,inode64 0 0
/dev/sda /run/rootfsbase ext4 ro,relatime 0 0
LiveOS_rootfs / overlay rw,relatime,lowerdir=/run/rootfsbase,upperdir=/run/overlayfs,workdir=/run/ovlwork,uuid=on,nouserxattr 0 0
LiveOS_rootfs /run/rootfsbase overlay rw,relatime,lowerdir=/run/rootfsbase,upperdir=/run/overlayfs,workdir=/run/ovlwork,uuid=on,nouserxattr 0 0
devtmpfs /dev devtmpfs rw,nosuid,size=159436k,nr_inodes=39859,mode=755,inode64 0 0
tmpfs /dev/shm tmpfs rw,nosuid,nodev,inode64,usrquota 0 0
devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=600,ptmxmode=000 0 0
sysfs /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
securityfs /sys/kernel/security securityfs rw,nosuid,nodev,noexec,relatime 0 0
cgroup2 /sys/fs/cgroup cgroup2 rw,nosuid,nodev,noexec,relatime,nsdelegate,memory_recursiveprot,memory_hugetlb_accounting 0 0
none /sys/fs/pstore pstore rw,nosuid,nodev,noexec,relatime 0 0
efivarfs /sys/firmware/efi/efivars efivarfs rw,nosuid,nodev,noexec,relatime 0 0
bpf /sys/fs/bpf bpf rw,nosuid,nodev,noexec,relatime,mode=700 0 0
configfs /sys/kernel/config configfs rw,nosuid,nodev,noexec,relatime 0 0
proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0
```

This breaks use cases where the underlying needs to be accessed.

Bug-Ubuntu: https://launchpad.net/bugs/2147471