build: Simplify build directory layout by putting all headers in genhdr.

Any generated headers go in $(BUILD)/genhdr/, and are #included as
'genhdr/xxx.h'.
diff --git a/py/qstr.h b/py/qstr.h
index 6bcb170..7cb6ae2 100644
--- a/py/qstr.h
+++ b/py/qstr.h
@@ -8,7 +8,7 @@
     MP_QSTR_NULL = 0, // indicates invalid/no qstr
     MP_QSTR_ = 1, // the empty qstr
 #define Q(id, str) MP_QSTR_##id,
-#include "build/py/qstrdefs.generated.h"
+#include "genhdr/qstrdefs.generated.h"
 #undef Q
     MP_QSTR_number_of,
 };