fix(health): give the LLDP phase a valid slot in the phase arrays

In HEALTH=1 builds health_phase(HEALTH_LLDP) used id 7 while ph_max[] and
ph_slow[] had HEALTH_PHASES = 6 entries. The LLDP checkpoint therefore
wrote one byte past ph_max (into the high byte of ph_slow[0]) and, when
the phase took two ticks or more, incremented two bytes past ph_slow
(dhcp_state.state and transaction_id on KP_9000_6XHML_X2).

Rename it HEALTH_PH_LLDP = 6, derive HEALTH_PHASES from it so the arrays
always cover the last phase, print an lldp line in "health" and document
it.

Fixes #658

rtlplayground.c

@@ -1203,7 +1203,7 @@ static void handle_tick(void)
 		lacp_timers();
 	health_phase(HEALTH_PH_STP);
 	lldp_tick();
-	health_phase(HEALTH_LLDP);
+	health_phase(HEALTH_PH_LLDP);
 	cabletest_tick();
 }