board/BuR: simplify default IP-setup on B&R boards.

To simplify and having a common default IP-setup on all B&R boards we
introduce an environment variable "brdefaultip" which does following.

Test if ${ipaddr} is empty, if yes it set's up some defaults:
- ipaddr   : 192.168.60.1
- netmask  : 255.255.255.0
- gatewayip: 192.168.60.254
- serverip : 192.168.60.254

This environment is ran from CONFIG_PREBOOT.

All other "tricks" are dropped.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>