Makes flush of IOs at exit configurable.

This change allows program which doesn't use the IO subsystem to
completelly get rid of it in the binary generated.

IO's are still flushed by default but it can be overriden in
configuration.

hal/common/retarget.cpp

@@ -609,9 +609,11 @@ extern "C" void exit(int return_code) {
 #endif
 
 #if DEVICE_STDIO_MESSAGES
+#if MBED_CONF_CORE_STDIO_FLUSH_AT_EXIT
     fflush(stdout);
     fflush(stderr);
 #endif
+#endif
 
 #if DEVICE_SEMIHOST
     if (mbed_interface_connected()) {

mbed_lib.json

@@ -9,6 +9,11 @@
         "stdio-baud-rate": {
             "help": "Baud rate for stdio",
             "value": 9600
+        },
+
+        "stdio-flush-at-exit": {
+            "help": "Enable or disable the flush of standard I/O's at exit.",
+            "value": true
         }
     }
 }