Add option to use USBSerial for stdio console

This commit introduces an option, `ep-atlas.enable-usb-stdio-console`, that will retarget the Mbed stdio console handle to a USBSerial instance if enabled.

Please note that if your application uses USB, it will conflict with this option. You should disable this option and implement a composite USB device in your application if you require stdio over USB.

This option is disabled by default so it will not cause issues with existing user code.

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/TARGET_EP_ATLAS/mbed_lib.json

@@ -10,6 +10,10 @@
             "help"                 : "Telit ME310C1 AT#PORTCFG Variant value",
             "macro_name"           : "EP_ATLAS_PORT_CONFIGURATION_VARIANT",
             "value"                : 0
+        },
+        "enable-usb-stdio-console": {
+            "help"                 : "Enables using USB Serial for the stdio console. If you use USB in your application, you must disable this feature and implement a composite USB device if you require USB serial output. This feature is disabled by default.",
+            "value"                : false
         }
     },
     "target_overrides": {
@@ -19,4 +23,4 @@
         }
 
     }
-}
+}

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/TARGET_MCU_NRF52840/TARGET_EP_ATLAS/usb_stdio.cpp

@@ -0,0 +1,34 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2021 ARM Limited
+ * Copyright (c) 2021 Embedded Planet, Inc.
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "USBSerial.h"
+#include "platform/mbed_retarget.h"
+
+#ifndef MBED_CONF_EP_ATLAS_ENABLE_USB_STDIO_CONSOLE
+#define MBED_CONF_EP_ATLAS_ENABLE_USB_STDIO_CONSOLE 0
+#endif
+
+#if MBED_CONF_EP_ATLAS_ENABLE_USB_STDIO_CONSOLE
+
+/* Retarget stdio to USBSerial */
+mbed::FileHandle *mbed::mbed_target_override_console(int fd) {
+    static USBSerial usb_serial;
+    return &usb_serial;
+}
+
+#endif