powerpc/motorola_powerpc: Add MVME2700 BSP

[chrisj at rtems.org]

Hi,

The MVME2700 can run the MVME2307 BSP. It is confusing in 
EPICS to have the MVME2700 referencing the mvme2307 BSP.
This patch adds the BSP powerpc/mvme2700.

The bsp.h header for the motorola_powerpc family needs the
RTEMS_BSP device to control the legacy network stack's
interface and attach defines. The rtems-net-legacy repo
needs those defines to build and run tests. The patch adds
the required defines for the mvme2307 and mvme2700.

Chris