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/unix/main.c b/unix/main.c
index e582244..b9b8fe0 100644
--- a/unix/main.c
+++ b/unix/main.c
@@ -22,7 +22,7 @@
 #include "runtime.h"
 #include "repl.h"
 #include "gc.h"
-#include "build/py/py-version.h"
+#include "genhdr/py-version.h"
 
 #if MICROPY_USE_READLINE
 #include <readline/readline.h>