zephyr: Add multi-image DFU for Thingy:53

Change enables multi-image DFU in Thingy:53 configuration.
The network core cannot access external flash directly. The flash
simulator must be used to provide a memory region that is used to
forward the new firmware to the network core.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>