kirkwood: rework Linksys EA[34]500 DTS

- Use board engineering names rather than marketing names

- Linksys uses a dual firmware layout, where the bootloader
  will switch to the other stored image when one fails to
  boot three consecutive times.

  In order to make this firmware compatible with the factory
  images and the stock bootloader we must match this layout.

Signed-off-by: Claudio Leite <leitec@staticky.com>

SVN-Revision: 47429