sim: Make BootGoResult into an enum
sim: Make BootGoResult into an enum

Now that this result is abstracted, take the special case for a sim stop
and make it its own field.  This hides the magic number we use to
indicate this to be entirely within mcuboot-sys.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot