Many thanks to Chris Zimman for the following suggestion which fixed my problem.
>> Do you have math emulation compiled into the kernel? Once this was configured the system boots up fine. I overlooked this as being the cause for several reasons:- 1) Using ebony_defconfig as the kernel config file should, I would have thought, set CONFIG_MATH_EMULATION=y but it does not. 2) I would not have thought my simple printf of some text would use floating point code. 3) I expected the kernel to panic with some form of illegal instruction trap if floating point instructions were being attempted with no emulator code present. Steve Boorman ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/