powerpc: Check that TASK_SIZE does not overlap KERNEL_START

Make sure CONFIG_TASK_SIZE does not overlap CONFIG_KERNEL_START
This could happen when overriding settings to get 1GB lowmem, and would lead
to userland mysteriousely hanging.

This setting is only used by PPC32.

Signed-off-by: Rune Torgersen <runet@innovsys.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>