Add fix for USB CDC ECM example

Additional delay is required to setup usb device before starting transmission.

Author: mprse

APIs_USB/USBCDC_ECM/README.md

@@ -3,9 +3,11 @@
 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.
+2. Use `dmesg -w` linux command to determine address of the USB CDC Ethernet device.
+3. Use `ifconfig` linux command to determine interface related to the the USB CDC Ethernet device.
+4. Open Wireshark.
+5. Click **Capture > Options** to select the correct capture interface.
+6. Click **Capture > Start**.
+7. 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.
+**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. It is suggested to run this example on Linux.

APIs_USB/USBCDC_ECM/main.cpp

@@ -20,6 +20,10 @@ USBCDC_ECM ecm;
 
 int main()
 {
+    // Let the USB device to setup
+    ThisThread::sleep_for(10);
+
+    // Send "Hello world" packets in loop
     while (true) {
         ecm.send((uint8_t *)&packet, sizeof(packet));
         ThisThread::sleep_for(1000);