boot/bootutil: Add MCUBOOT_SINGLE_APPLICATION_SLOT_RAM_LOAD mode

Following the split of RAM code, these definitions will help use it with
single slot applications.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>
Signed-off-by: Tom Burdick <thomas.burdick@intel.com>
(cherry picked from commit 5c67fb99)
Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>