To summarize, I asked Christoph off list (because it was
late at night and I didn't want him to feel forgotten) to check the
kernel startup output from running ./aboriginal-start --interactive
from the aboriginal-controller wrapper scripts to further debug the
problem, and he has also been trying to add support for ARMv7-a
float to aboriginal.
meanwhile I compiled the ARMv5l image again from scratch and added the --interactive
option to the Python script in YBD (sandbox.py) that calls aboriginal-start.
This time I saw the boot messages of the kernel and finally got a shell prompt. So this
seems to be working.
Regarding Cortex-A15 I suspect that a device tree is needed to bring up this platform.