target-openrisc: Machine improvements take two

Hi,

Because of the wrapping errors in the previous set here is the set again.

The purpose is to match the jor1k in functionality.
For testing this vmlinux should work http://mrs.ro/dl/or1k/vmlinux and
the hd images from jor1k(hdX is mirrored here http://mrs.ro/dl/or1k/hdX)

Regards,
Valentin