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.

rtlplayground.c

@@ -1004,7 +1004,7 @@ void handle_rx(void)
 			REG_SET(RTL837X_REG_NIC_RXCMD, 1);
 			return;
 		}
-		health_rx_frames++;
+		health_rx_frame();
 
 #ifdef RXTXDBG
 		print_string("\n<< ");