native soc: Add option to pass extra images to native simulator build

Add a new kconfig option to be able to pass extra images to the
native simulator build.
So one can, for ex., use one application build to produce one core image,
and at the same time have it produce the final link with the native
simulator runner and the other MCU images.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>