AP_HAL_Linux: 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_Linux/HAL_Linux_Class.cpp

@@ -491,3 +491,8 @@ const AP_HAL::HAL &AP_HAL::get_HAL()
 {
     return hal_linux;
 }
+
+AP_HAL::HAL &AP_HAL::get_HAL_mutable()
+{
+    return hal_linux;
+}