Add asynchronous serial

Author: svastm

hal/targets/hal/TARGET_STM/TARGET_STM32F2/objects.h

@@ -74,6 +74,14 @@ struct serial_s {
     uint32_t parity;
     PinName pin_tx;
     PinName pin_rx;
+#if DEVICE_SERIAL_ASYNCH
+    uint32_t events;
+#endif
+#if DEVICE_SERIAL_FC
+    uint32_t hw_flow_ctl;
+    PinName pin_rts;
+    PinName pin_cts;
+#endif
 };
 
 struct spi_s {