Add cpu.h/cpp, avr subdir

The idea is to have a separate implementation of the HAL interface for each CPU. So far we only have the AVR.

src/hal/avr/cpu.cpp

@@ -0,0 +1,10 @@
+#include "../cpu.h"
+
+namespace hal {
+namespace CPU {
+
+    void Init() {
+    }
+
+} // namespace CPU
+} // namespace hal

src/hal/cpu.h

@@ -0,0 +1,12 @@
+#pragma once
+
+/// Hardware Abstraction Layer for the CPU
+
+namespace hal {
+namespace CPU {
+
+    /// CPU init routines (not really necessary for the AVR)
+    void Init();
+
+} // namespace CPU
+} // namespace hal