openwrt: Default to iptables-nft

OpenWrt is moving to using nftables by default, but the 'iptables' binary
is likely to stick around for a while. So add a platform-specific
preference for using iptables-nft (if it exists). We'll still fall back to
'iptables' if 'iptables-nft' is not installed on the system, and we'll
still get the warning if neither exists and simple.qos is used.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

platform/openwrt/sqm.conf

@@ -3,3 +3,5 @@ SQM_STATE_DIR=/var/run/sqm
 SQM_QDISC_STATE_DIR=${SQM_STATE_DIR}/available_qdiscs
 SQM_CHECK_QDISCS="fq_codel codel pie sfq cake"
 SQM_SYSLOG=1
+IP6TABLES_BINARY=$(command -v ip6tables-nft)
+IPTABLES_BINARY=$(command -v iptables-nft)