GCS_MAVLink: increase statustext queue size to 7 on low-mem boards

... and SITL....

Copter currently spits out so many messages on a banner-send (e.g what
we do when parameters are fetched) that it puses the first sent message
straight out of the queue before it gets a chance to be sent from the
queue.

libraries/GCS_MAVLink/GCS.cpp

@@ -12,6 +12,12 @@
 
 extern const AP_HAL::HAL& hal;
 
+// if this assert fails then fix it and the comment in GCS.h where
+// _statustext_queue is declared
+#if CONFIG_HAL_BOARD == HAL_BOARD_CHIBIOS
+assert_storage_size<GCS::statustext_t, 58> _assert_statustext_t_size;
+#endif
+
 void GCS::get_sensor_status_flags(uint32_t &present,
                                   uint32_t &enabled,
                                   uint32_t &health)

libraries/GCS_MAVLink/GCS.h

@@ -1035,12 +1035,13 @@ private:
 
     void service_statustext(void);
 #if HAL_MEM_CLASS <= HAL_MEM_CLASS_192 || CONFIG_HAL_BOARD == HAL_BOARD_SITL
-    static const uint8_t _status_capacity = 5;
+    static const uint8_t _status_capacity = 7;
 #else
     static const uint8_t _status_capacity = 30;
 #endif
 
-    // queue of outgoing statustext messages
+    // queue of outgoing statustext messages.  Each entry consumes 58
+    // bytes of RAM on stm32
     StatusTextQueue _statustext_queue{_status_capacity};
 
     // true if we have already allocated protocol objects: