[bbr] add a flag to enable BBR on init (#2265)

Flag `OTBR_ENABLE_BACKBONE_ROUTER_ON_INIT` is created for enabling BBR
on init. It's set to OTBR_ENABLE_BACKBONE_ROUTER by default and this
works when infra link is statically configured. If infra link is
dynamically configured then this flag should be set to 0, the
component that sets infra link should enable/disable BBR feature with
the otBackboneRouterSetEnabled API.

src/backbone_router/backbone_agent.cpp

@@ -71,7 +71,9 @@ void BackboneAgent::Init(void)
     mNdProxyManager.Init();
 #endif
 
+#if OTBR_ENABLE_BACKBONE_ROUTER_ON_INIT
     otBackboneRouterSetEnabled(mNcp.GetInstance(), /* aEnabled */ true);
+#endif
 }
 
 void BackboneAgent::HandleThreadStateChanged(otChangedFlags aFlags)

src/backbone_router/backbone_agent.hpp

@@ -36,6 +36,10 @@
 
 #include "openthread-br/config.h"
 
+#ifndef OTBR_ENABLE_BACKBONE_ROUTER_ON_INIT
+#define OTBR_ENABLE_BACKBONE_ROUTER_ON_INIT OTBR_ENABLE_BACKBONE_ROUTER
+#endif
+
 #if OTBR_ENABLE_BACKBONE_ROUTER
 
 #include <openthread/backbone_router_ftd.h>