boot/zephyr: Add CONFIG_SINGLE_APPLICATION_SLOT_RAM_LOAD

This option basically enables MCUBOOT_RAM_LOAD in a single
slot configuration, meaning the image on slot0 will be loaded into RAM.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>
Signed-off-by: Tom Burdick <thomas.burdick@intel.com>
(cherry picked from commit 249a71d1)
Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>