system-macros: Create ADD_BR variant.

system-macros: Create ADD_BR variant.

This patch splits ADD_BR into two commands, so they can be used from
different contexts:

ADD_BR(...) is a standalone command to add a bridge to OVS, and allows
additional ovs-vsctl arguments to be passed. It uses _ADD_BR().
_ADD_BR(...) is the implementation-specific ovs-vsctl arguments to
set up the correct datapath type for userspace or kmod tests.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>