Sub: add available memory to PM message

ArduSub/Log.cpp

@@ -295,6 +295,7 @@ struct PACKED log_Performance {
     uint8_t i2c_lockup_count;
     uint16_t ins_error_count;
     uint32_t log_dropped;
+    uint32_t mem_avail;
 };
 
 // Write a performance monitoring packet
@@ -310,6 +311,7 @@ void Sub::Log_Write_Performance()
         i2c_lockup_count : 0,
         ins_error_count  : ins.error_count(),
         log_dropped      : DataFlash.num_dropped() - perf_info_get_num_dropped(),
+        hal.util->available_memory()
     };
     DataFlash.WriteCriticalBlock(&pkt, sizeof(pkt));
 }
@@ -585,7 +587,7 @@ const struct LogStructure Sub::log_structure[] = {
     { LOG_CONTROL_TUNING_MSG, sizeof(log_Control_Tuning),
       "CTUN", "Qffffffeccfhh", "TimeUS,ThrIn,ABst,ThrOut,DAlt,Alt,BAlt,DSAlt,SAlt,TAlt,DCRt,CRt" },
     { LOG_PERFORMANCE_MSG, sizeof(log_Performance), 
-      "PM",  "QHHIhBHI",    "TimeUS,NLon,NLoop,MaxT,PMT,I2CErr,INSErr,LogDrop" },
+      "PM",  "QHHIhBHII",    "TimeUS,NLon,NLoop,MaxT,PMT,I2CErr,INSErr,LogDrop,Mem" },
     { LOG_MOTBATT_MSG, sizeof(log_MotBatt),
       "MOTB", "Qffff",  "TimeUS,LiftMax,BatVolt,BatRes,ThLimit" },
     { LOG_EVENT_MSG, sizeof(log_Event),