Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

core/os/keyboard.cpp

@@ -29,7 +29,8 @@
 /*************************************************************************/
 
 #include "keyboard.h"
-#include "os/os.h"
+
+#include "core/os/os.h"
 
 struct _KeyCodeText {
 	int code;