AP_Logger: added get_log_start_count()

used by UAVCAN to know if it should log CAN devices on log rotation

libraries/AP_Logger/AP_Logger.cpp

@@ -580,6 +580,8 @@ void AP_Logger::Write_MessageF(const char *fmt, ...)
 
 void AP_Logger::backend_starting_new_log(const AP_Logger_Backend *backend)
 {
+    _log_start_count++;
+
     for (uint8_t i=0; i<_next_backend; i++) {
         if (backends[i] == backend) { // pointer comparison!
             // reset sent masks

libraries/AP_Logger/AP_Logger.h

@@ -408,6 +408,11 @@ public:
     // output a FMT message for each backend if not already done so
     void Safe_Write_Emit_FMT(log_write_fmt *f);
 
+    // get count of number of times we have started logging
+    uint8_t get_log_start_count(void) const {
+        return _log_start_count;
+    }
+
 protected:
 
     const struct LogStructure *_structures;
@@ -531,6 +536,10 @@ private:
     // last time arming failed, for backends
     uint32_t _last_arming_failure_ms;
 
+    // count of number of times we've started logging
+    // can be used by other subsystems to detect if they should log data
+    uint8_t _log_start_count;
+
     bool should_handle_log_message();
     void handle_log_message(class GCS_MAVLINK &, const mavlink_message_t &msg);