stream: Make non-blcoking stream support configurable.

Enable only on unix. To avoid unpleasant surprises with error codes.
diff --git a/unix/mpconfigport.h b/unix/mpconfigport.h
index b15fa63..41bc6a3 100644
--- a/unix/mpconfigport.h
+++ b/unix/mpconfigport.h
@@ -39,6 +39,7 @@
 #define MICROPY_FLOAT_IMPL          (MICROPY_FLOAT_IMPL_DOUBLE)
 #define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_MPZ)
 #define MICROPY_PATH_MAX            (PATH_MAX)
+#define MICROPY_STREAMS_NON_BLOCK   (1)
 #define MICROPY_USE_COMPUTED_GOTO   (1)
 #define MICROPY_MOD_SYS_STDFILES    (1)
 #define MICROPY_ENABLE_MOD_CMATH    (1)