[nit] fix BACKBONE_ROUTER readonly warning (#623)
[nit] fix BACKBONE_ROUTER readonly warning (#623)


openthread/ot-br-posix,bukepo/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,bukepo/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,bukepo/borderrouter,bukepo/borderrouter,openthread/borderrouter,openthread/ot-br-posix,bukepo/borderrouter,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix