[border-router] send at least one router solicitation (#6280)
[border-router] send at least one router solicitation (#6280)


abtink/openthread,srickardti/openthread,jwhui/openthread,srickardti/openthread,jwhui/openthread,openthread/openthread,srickardti/openthread,abtink/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread,jwhui/openthread,openthread/openthread,openthread/openthread,abtink/openthread,openthread/openthread