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.c

@@ -0,0 +1,21 @@
+#include "rtl837x_common.h"
+#include "rtl837x_sfr.h"
+#include "rtl837x_regs.h"
+#include "rtl837x_trap.h"
+
+#pragma codeseg BANK2
+#pragma constseg BANK2
+
+extern __xdata uint8_t sfr_data[4];
+
+void trap_init(void) __banked
+{
+	REG_SET(RTL837X_EXT_CPU_CTRL, CPU_PORT);
+}
+
+void trap_rma_set(uint16_t reg, uint8_t act) __banked
+{
+	reg_read_m(reg);
+	sfr_data[3] = (sfr_data[3] & ~RTL837X_RMA_ACT_MASK) | act;
+	reg_write_m(reg);
+}