refactoring machine.c, two files per device

machine/default_machine.h

@@ -0,0 +1,17 @@
+#ifndef _MACHINE_DEFAULT_H_
+#define _MACHINE_DEFAULT_H_
+
+/*
+ * Fallback selected by machine.h when MACHINE_NAME is not defined on the
+ * command line. Firmware builds always pass -DMACHINE_NAME=<MACHINE>, so this
+ * is used by translation units that only need the machine.h declarations --
+ * notably the host-side tests under test/, which define their own `machine`
+ * object.
+ *
+ * MACHINE_MODEL is deliberately not defined here: a build that reaches this
+ * header must not silently pick up some default model string. Any code that
+ * needs MACHINE_MODEL therefore fails to compile unless a real machine is
+ * selected.
+ */
+
+#endif