stp: full RSTP and a reworked Spanning Tree page

* stp: select the root port by cost and block redundant links
* stp: move the protocol to BANK4 and keep the command line in BANK2
* stp: implement the RSTP state machines of 802.1D-2004 clause 17
* stp: counters, more settings and a fuller Spanning Tree page
* stp: make a link aggregation group one port of the RSTP tree
* web: serve the Spanning Tree JSON from BANK4 as well

Kept in BANK1, the larger Spanning Tree page would leave under 1 kB
there for the rest of the web server and every page still to come, so
send_stp() and send_stp_counters() move to httpd/page_stp.c in BANK4
with the protocol, as banked calls, with their own copies of the few output
helpers and of the JSON header, so nothing in BANK4 calls into BANK1
directly or hands it a pointer into its code space. BANK4 takes the
free flash between BANK3 and the web files.

KP_9000_6XHML_X2: BANK1 44336, BANK2 36969, BANK3 11942, BANK4 20965
bytes of 49152.

test/stp/Makefile

@@ -0,0 +1,21 @@
+CC ?= cc
+CFLAGS = -std=c11 -Wall -Wextra -Wno-unknown-pragmas \
+         -D__xdata= -D__code= -D__banked= -D__reentrant= \
+         -Ishim -I../..
+
+all: build/stp_sim
+	./build/stp_sim
+
+verbose: build/stp_sim
+	./build/stp_sim -v
+
+build/stp_sim: stp_sim.c ../../rtl837x_stp.c ../../rtl837x_stp_cli.c ../../rtl837x_stp.h shim/*.h
+	mkdir -p build
+	cp ../../rtl837x_stp.c build/stp_under_test.c
+	cp ../../rtl837x_stp_cli.c build/stp_cli_under_test.c
+	$(CC) $(CFLAGS) -o $@ stp_sim.c build/stp_under_test.c build/stp_cli_under_test.c
+
+clean:
+	rm -rf build
+
+.PHONY: all verbose clean