trap: deliver reserved multicast to the CPU by trap, STP first

A reserved-multicast trap goes to the port named in EXT_CPU_CTRL, which
names no port out of reset, so trapped frames were dropped at ingress.
trap_init() points it at the CPU port once at startup, and
trap_rma_set() changes only the action field of an RMA register.

A trapped frame arrives without the VLAN tag a forwarded one gets, so
TRAP_RX_BODY() gives the offset of the payload for both forms.

STP now traps its address while it runs instead of steering it with a
static L2 entry per VLAN, so turning it on no longer walks the VLAN
table and a pvid change no longer needs stp off and on. With STP off
the flooding entries are restored as before.

A trapped BPDU passes the tagged-only ingress filter that drops a
forwarded one, so the stp_setup() warning about such ports goes.

rtl837x_trap.h

@@ -0,0 +1,16 @@
+#ifndef _RTL837X_TRAP_H_
+#define _RTL837X_TRAP_H_
+
+#include <stdint.h>
+#include "rtl837x_common.h"
+
+#define TRAP_RX_HDR_SIZE	(12 + RTL_TAG_SIZE)
+
+#define TRAP_RX_BODY() \
+	((uip_buf[TRAP_RX_HDR_SIZE] == 0x81 && uip_buf[TRAP_RX_HDR_SIZE + 1] == 0x00) \
+	 ? TRAP_RX_HDR_SIZE + VLAN_TAG_SIZE : TRAP_RX_HDR_SIZE)
+
+void trap_init(void) __banked;
+void trap_rma_set(uint16_t reg, uint8_t act) __banked;
+
+#endif