Avoid wait_ms() spin

System's wait_ms() spins to achieve a precise delay - we don't want this.
Call Thread::wait directly.

drivers/UARTSerial.cpp

@@ -19,7 +19,12 @@
 #include <errno.h>
 #include "UARTSerial.h"
 #include "platform/mbed_poll.h"
+
+#if MBED_CONF_RTOS_PRESENT
+#include "rtos/Thread.h"
+#else
 #include "platform/mbed_wait_api.h"
+#endif
 
 namespace mbed {
 
@@ -277,6 +282,17 @@ void UARTSerial::tx_irq(void)
     }
 }
 
+void UARTSerial::wait_ms(uint32_t millisec)
+{
+    /* wait_ms implementation for RTOS spins until exact microseconds - we
+     * want to just sleep until next tick.
+     */
+#if MBED_CONF_RTOS_PRESENT
+    rtos::Thread::wait(millisec);
+#else
+    ::wait_ms(millisec);
+#endif
+}
 } //namespace mbed
 
 #endif //(DEVICE_SERIAL && DEVICE_INTERRUPTIN)

drivers/UARTSerial.h

@@ -160,6 +160,8 @@ public:
 
 private:
 
+    void wait_ms(uint32_t millisec);
+
     /** SerialBase lock override */
     virtual void lock(void);