powerpc/85xx: use one kernel option for all the CoreNet_Generic boards

Currently all these boards use the same machine struct and also select
the same kernel options, so it seems a bit of redundant to keep one
separate kernel option for each board. Also update the defconfigs
according to this change.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>