use the toolchain's default CPU (464fp) as the CPU option.
This fixes a CPU selection prompt which shows up now.
CPU selection
> 1. Generic 32 bits powerpc (POWERPC_CPU) (NEW)
2. Rely on the toolchain's implicit default CPU (TOOLCHAIN_DEFAULT_CPU) (NEW)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>