powerpc/corenet_ds: Various updates to initial env cfg

* Make the U-Boot update command sequence conditional.  Helps prevent
  accidental erasing if an upload or previous step fails
* Make it easier to update other FLASH banks
* Enable DDR controller cache line interleaving and bank cs0/cs1 by default

Signed-off-by: Emil Medve <Emilian.Medve@Freescale.com>
Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>