health: build the instrumentation only when asked for

The counters, the phase hooks in idle() and the "health" command are now
behind HEALTH, set from the command line with HEALTH=1. Without it the
hooks are empty macros and the command does not exist, so a default build
carries none of it: on SWTGW218AS that is 1065 bytes of bank 2, 101 of the
common segment and 28 of xdata, plus seven banked calls per main-loop pass.

It is a developer's tool. Reading the dump takes knowing the main loop, and
a user who needs it for a report can build with the flag on, which is how
the kernel treats its lock and latency instrumentation as well.

doc/health.md

@@ -20,6 +20,17 @@ tcp st 00 tmr 00 rtx 00 rto 00 len 0x0000 mss 0x05b4
 httpd left 0x0000 entry 00 stp 01 mvlan 0x0002
 ```
 
+The instrumentation is not part of a default build. It costs about 1.1 kB
+of bank 2, a few bytes of xdata and seven banked calls per main-loop pass,
+which is nothing a user needs unless they are chasing a problem, so it is
+switched on at build time:
+
+```
+make MACHINE=SWTGW218AS HEALTH=1
+```
+
+Without it the hooks compile to nothing and `health` is not a command.
+
 All figures are raw hex. The counters are cumulative: take two dumps a known
 time apart and the differences give the rates.