Karl:

>  The first one gives me an unbootable system
> $ ldd /sbin/init | grep /usr
>         libpcre2-8.so.0 => /usr/lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x00007f737ba28000)

>
> fortunately /bin/bash didn't depend on /usr so I could boot with
> init=/bin/bash, but there is more breakage:
...

booting with init=/bin/bash and running the files /etc/rcS.d/S* manually
it seems to be fsck.* and mount who has dependacies on /usr.

To fix this, theese packages needs to un-/usr-merged:
libmount1
libblkid1
libpcre2-8-0
libuuid1
uuid-dev

busybox can replace the mount command, but there is no fsck.* in
busybox.

Regards,
/Karl Hammar