AP_BattMonitor: make healthy() check all configured monitors

GCS reporting for SYS_STATUS should check all healthy, not just first
backend

libraries/AP_BattMonitor/AP_BattMonitor.cpp

@@ -740,6 +740,19 @@ uint32_t AP_BattMonitor::get_mavlink_fault_bitmask(const uint8_t instance) const
     return drivers[instance]->get_mavlink_fault_bitmask();
 }
 
+/*
+  check that all configured battery monitors are healthy
+ */
+bool AP_BattMonitor::healthy() const
+{
+    for (uint8_t i=0; i< _num_instances; i++) {
+        if (get_type(i) != Type::NONE && !healthy(i)) {
+            return false;
+        }
+    }
+    return true;
+}
+
 namespace AP {
 
 AP_BattMonitor &battery()

libraries/AP_BattMonitor/AP_BattMonitor.h

@@ -156,7 +156,9 @@ public:
 
     // healthy - returns true if monitor is functioning
     bool healthy(uint8_t instance) const;
-    bool healthy() const { return healthy(AP_BATT_PRIMARY_INSTANCE); }
+
+    // return true if all configured battery monitors are healthy
+    bool healthy() const;
 
     /// voltage - returns battery voltage in volts
     float voltage(uint8_t instance) const;