Move SET_MACHINE_COMPAT macro to boards.h

pc.h defines a SET_MACHINE_COMPAT macro to make setting up compat_props
for the various PC machine versions less verbose.  There's nothing
inherently PC specific about it, though, so move it to boards.h where other
versioned machine types (like pseries-*) can use it.

While we're doing that, change it's indentation to be a bit more regular.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>

include/hw/boards.h

@@ -157,4 +157,13 @@ struct MachineState {
     } \
     machine_init(machine_initfn##_register_types)
 
+#define SET_MACHINE_COMPAT(m, COMPAT) \
+    do {                              \
+        static GlobalProperty props[] = {       \
+            COMPAT                              \
+            { /* end of list */ }               \
+        };                                      \
+        (m)->compat_props = props;              \
+    } while (0)
+
 #endif