zephyr: Wrapped the RAM loading mode in Zephyr configs.

The RAM loading mode wasn't wrapped in Zephyr configs like the other boot modes.
Added a config to enable RAM loading as well as two configs to set
IMAGE_EXECUTABLE_RAM_START and IMAGE_EXECUTABLE_RAM_SIZE, respectively. These
two values default to the values from the chosen node sram in the dts.

Signed-off-by: Johan Öhman <johan.ohman@softube.com>