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.c b/py/qstr.c
index 2b14065..f9f63da 100644
--- a/py/qstr.c
+++ b/py/qstr.c
@@ -60,7 +60,7 @@
         (const byte*) "\0\0\0\0", // invalid/no qstr has empty data
         (const byte*) "\0\0\0\0", // empty qstr
 #define Q(id, str) str,
-#include "build/py/qstrdefs.generated.h"
+#include "genhdr/qstrdefs.generated.h"
 #undef Q
     },
 };