boot/zephyr: Load image to RAM on single loader

Now that's possible to load image to RAM on single loaders, add support
on Zephyr port for that.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>
Signed-off-by: Tom Burdick <thomas.burdick@intel.com>
(cherry picked from commit 48931935)
Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>