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.

rtlplayground.c

@@ -11,6 +11,7 @@
 #include "rtl837x_pins.h"
 #include "rtl837x_phy.h"
 #include "rtl837x_port.h"
+#include "rtl837x_trap.h"
 #include "rtl837x_stp.h"
 #include "rtl837x_igmp.h"
 #include "rtl837x_leds.h"
@@ -1793,6 +1794,7 @@ void main(void)
 	stp_defaults();		/* 802.1D/w default config before any "stp ..." replay */
 	nic_setup();
 	REG_SET(RTL837X_NIC_INT_MSK, NIC_INT_RXIE);
+	trap_init();
 	vlan_setup();
 	port_l2_setup();
 	igmp_setup();