board/BuR/kwb: use bootvx(...) (with bootline feature) instead go(...)

Since we don't have for sure a valid IP-setup during
board_late_init(...) because it maybe allready stored in environment or
not, we cannot form a proper vxWorks bootline at this place.

So we move to the way, forming the bootline just before
executing/launching vxWorks. To do this we use the bootvx command
instead go.

We only have to form the "othbootargs" environment variable, the rest is
done pretty good by the "bootvx" commannd.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>