stmhal: Implement sys.std{in,out,err}.buffer, for raw byte mode.

It's configurable and only enabled for stmhal port.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 560fdce..5065e36 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -594,6 +594,11 @@
 #define MICROPY_PY_SYS_STDFILES (0)
 #endif
 
+// Whether to provide sys.{stdin,stdout,stderr}.buffer object
+// This is implemented per-port
+#ifndef MICROPY_PY_SYS_STDIO_BUFFER
+#define MICROPY_PY_SYS_STDIO_BUFFER (0)
+#endif
 
 // Extended modules