boot/zephyr: Use MCUBOOT_RAM_LOAD instead of CONFIG_BOOT_RAM_LOAD

This will encompass both CONFIG_BOOT_RAM_LOAD and
CONFIG_SINGLE_APPLICATION_SLOT_RAM_LOAD, which, at this point, should
follow the same code path - load image to RAM.

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