overlayfs: Allow not used mount points

When machine configuration defines a mount point, which is not used in
any recipe, allow to fall through and only report a note in the logs.
This can be expected behavior, when a mount point is defined for several
machines, but not used in all of them

Signed-off-by: Vyacheslav Yurkov <Vyacheslav.Yurkov@bruker.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>