Add USBCDC_ECM example

USB/USBCDC_ECM/README.md

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+## USBCDC_ECM example
+
+The example sends an Ethernet frame that carries "Hello world" payload with a custom EtherType to the host PC. You can capture the frame by using a program called "Wireshark":
+
+1. Flash the board, and ensure the target's USB is plugged into the PC.
+2. Open Wireshark.
+3. Click **Capture > Options** to select the correct capture interface.
+4. Click **Capture > Start**.
+5. Click captured packet from source address 12:34:56:78:9a:bc to see the "Hello world" payload.
+
+**Note:** Because Windows doesn't provide native support for the USB CDC-ECM model, you must use third party drivers to use this class on Windows.

USB/USBCDC_ECM/main.cpp

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+/*
+* Copyright (c) 2019 ARM Limited. All rights reserved.
+* 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 "mbed.h"
+#include "USBCDC_ECM.h"
+
+/* Ethernet II frame */
+typedef struct {
+    uint8_t dst_mac[6];
+    uint8_t src_mac[6];
+    uint16_t eth_type;
+    char payload[12];
+} packet_t;
+
+static packet_t packet = {
+    .dst_mac = {0xff, 0xff, 0xff, 0xff, 0xff, 0xff},
+    .src_mac = {0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc},
+    .eth_type = 0xaaaa, /* unused EtherType */
+    .payload = "Hello world"
+};
+
+USBCDC_ECM ecm;
+
+int main()
+{
+    while (true) {
+        ecm.send((uint8_t *)&packet, sizeof(packet));
+        wait(1.0);
+    }
+}

USB/USBCDC_ECM/mbed-os.lib

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+https://github.com/ARMmbed/mbed-os/#0063e5de32fc575f061244c96ac60c41c07bd2e6