boot/zephyr: Fix SINGLE_APPLICATION_SLOT_RAM_LOAD file inclusion

Tweak if/else chain so that:
  - single_loader.c and ram_load.c are included if
    CONFIG_SINGLE_APPLICATION_SLOT_RAM_LOAD=y.
  - single_loader.c is included if CONFIG_SINGLE_APPLICATION_SLOT=y
  - ram_load.c is included if CONFIG_BOOT_RAM_LOAD=y and
    CONFIG_SINGLE_APPLICATION_SLOT_RAM_LOAD=n and
    CONFIG_SINGLE_APPLICATION_SLOT=n.

Without this patch, having both CONFIG_SINGLE_APPLICATION_SLOT_RAM_LOAD
and CONFIG_SINGLE_APPLICATION_SLOT would not include ram_load.c.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>