sim: Enable sim building with ram-load

Allow the sim to test configurations with MCUBOOT_RAM_LOAD defined.
This does not define any tests for this mode.

This adds definitions for simulated RAM offsets to the simulator.  This
will define the offset (from `IMAGE_RAM_BASE`) to where the simulated
RAM will be located.  For now, just give these somewhat reasonable
values so that we are able to at least compile the RAM_LOADING code in
the simulator.

Signed-off-by: David Brown <david.brown@linaro.org>