Current way of handling overlay-manager links is a bit strange: each
manager has a static array, containing pointers to all the overlays
(even those used by other managers). The overlays contain a pointer to
the manager being used.

This patch makes the system a bit saner: each manager has a linked list
of overlays, and only the overlays linked to that manager are in the
list.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>