Create a new file, apply.c, and move code about handling the
apply-mechanism and configuration of the managers and overlays from
manager.c to apply.c.

Not all related code is moved in this patch, but only the core
apply/configure functions. The later patches move rest of the code from
overlay.c and manager.c, adding necessary locking at the same time.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>