yocto/kas/systemready-ir: Build zephyr-helloworld SAFETY_ISLAND_IMAGE

Building zephyr-helloworld as the SAFETY_ISLAND_IMAGE improves the SystemReady
IR ACS validation time because the Safety Island will not be exectuing any
instructions.

Issue-Id: SCM-5314
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: I24fe8dd2c4b381f56eccd56ede0fb7fcdd54c056