AP_Logger: add AP::logger() method

libraries/AP_Logger/AP_Logger.cpp

@@ -1036,3 +1036,8 @@ bool AP_Logger::Log_Write_ISBD(const uint16_t isb_seqno,
 
     return backends[0]->WriteBlock(&pkt, sizeof(pkt));
 }
+
+AP_Logger &AP::logger()
+{
+    return *AP_Logger::instance();
+}

libraries/AP_Logger/AP_Logger.h

@@ -385,3 +385,7 @@ private:
     /* end support for retrieving logs via mavlink: */
 
 };
+
+namespace AP {
+    AP_Logger &logger();
+};