*facepalm* Thank you. I should have seen that. That example is now working as expected.
As another minor problem, your run script misses to build 'core' and 'lib/ld'. The respective 'core' and 'ld.lib.so' boot modules are needed too. Nowadays, we tend to use the 'build_artifacts' function [2] to foster the consistency between the 'build' and 'build_boot_image' arguments. I recommend changing the line to: build_boot_image [build_artifacts]
Noted, and thank you. That makes the build_boot_image a lot simpler.