Lots of tidying

- Removed generic `avr8` target
- Simplified AVR8 target construction
- Introduced register descriptor IDs
- Simplified GDB register mappings
- Simplified target interface contract
- Other bits of tidying

src/DebugToolDrivers/Microchip/XplainedPro/XplainedPro.hpp

@@ -27,6 +27,7 @@ namespace Bloom::DebugToolDrivers
             return "Xplained Pro";
         }
 
-        void init() override;
+    protected:
+        void configureAvr8Interface() override;
     };
 }