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/MplabPickit4/MplabPickit4.cpp

@@ -18,7 +18,6 @@ namespace Bloom::DebugToolDrivers
 
         try {
             EdbgDevice::init();
-            this->edbgAvr8Interface->setReactivateJtagTargetPostProgrammingMode(true);
 
         } catch (const DeviceNotFound& exception) {
             /*
@@ -40,4 +39,8 @@ namespace Bloom::DebugToolDrivers
             throw exception;
         }
     }
+
+    void MplabPickit4::configureAvr8Interface() {
+        this->edbgAvr8Interface->setReactivateJtagTargetPostProgrammingMode(true);
+    }
 }