AP_HAL: introduce get_HAL_mutable() to complement get_HAL()

Returns a mutable reference to the same HAL for certain purposes where
the HAL needs to be mutated to avoid UB problems with casting away const
and to make the fact that mutation is happening obvious.

libraries/AP_HAL/AP_HAL_Namespace.h

@@ -67,6 +67,7 @@ namespace AP_HAL {
 
     class SIMState;
 
-    // Must be implemented by the concrete HALs.
+    // Must be implemented by the concrete HALs and return the same reference.
     const HAL& get_HAL();
+    HAL& get_HAL_mutable();
 }