deploy: Support an empty `/etc` and populated `/usr/etc`

deploy: Support an empty `/etc` and populated `/usr/etc`

In preparation for support for a transient `/etc`:
https://github.com/ostreedev/ostree/issues/2868
particularly in combination with composefs.

Basically it's just much more elegant if we can directly mount
an overlayfs on the *empty* `etc` directory, using `usr/etc` as
the lower.

In the composefs case, we'd have to mount the composefs overlayfs
itself writable (and call `mkdir`) *just* so we can make that
empty `etc` directory which is ugly.