odhcpd: log “Sending a RA on lan” at LOG_DEBUG

This reduces log spam when dhcp.odhcpd.loglevel ≥ 5, as neighbor
discovery router advertisements are routinely generated by timer and in
response to router solicitations.

Patch submitted upstream at openwrt/odhcpd#191.

package/network/services/odhcpd/patches/100-log_sending_ra_at_debug.patch

@@ -0,0 +1,13 @@
+diff --git a/src/router.c b/src/router.c
+index eca0bf7a2822..504cbed6dee4 100644
+--- a/src/router.c
++++ b/src/router.c
+@@ -782,7 +782,7 @@ static int send_router_advert(struct interface *iface, const struct in6_addr *fr
+ 	else
+ 		inet_pton(AF_INET6, ALL_IPV6_NODES, &dest.sin6_addr);
+
+-	syslog(LOG_NOTICE, "Sending a RA on %s", iface->name);
++	syslog(LOG_DEBUG, "Sending a RA on %s", iface->name);
+
+ 	if (odhcpd_send(iface->router_event.uloop.fd, &dest, iov, ARRAY_SIZE(iov), iface) > 0)
+ 		iface->ra_sent++;