buildkernel -c --stage-only
is solving issue of dis-rebuilt kernel, which is compromising Plymouth base theme, namely referring to NHS soft systems methodology in action for the last 30+ years. Kernel is built successfully, consecutively. Further on, based on 2 flash keys, dropped on a desk nearby at hand, is issued:
with the consecutive flash keys in a queue. Kernel is then copied from staged folder to a ready to use flash key, making it boot EFI device.