sim: Fixup devicetree.h for changes to mcuboot

Fixup how we fake out devicetree.h, which is now really the flash map
API for what mcuboot is using.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>