add CAN example (#101)

Author: maciejbocianski

APIs_Drivers/CAN_ex_1/README.md

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+# CAN example
+
+This example sends a counter from one CAN bus (can1) and listens for a packet on the other CAN bus (can2).
+Each bus controller should be connected to a CAN bus transceiver.
+These should be connected together at a CAN bus.

APIs_Drivers/CAN_ex_1/main.cpp

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+/*
+ * Copyright (c) 2017-2020 Arm Limited and affiliates.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#if !DEVICE_CAN
+#error [NOT_SUPPORTED] CAN not supported for this target
+#endif
+
+
+#include "mbed.h"
+
+
+Ticker ticker;
+DigitalOut led1(LED1);
+DigitalOut led2(LED2);
+/** The constructor takes in RX, and TX pin respectively.
+  * These pins, for this example, are defined in mbed_app.json
+  */
+CAN can1(MBED_CONF_APP_CAN1_RD, MBED_CONF_APP_CAN1_TD);
+CAN can2(MBED_CONF_APP_CAN2_RD, MBED_CONF_APP_CAN2_TD);
+char counter = 0;
+
+void send()
+{
+    printf("send()\n");
+    if (can1.write(CANMessage(1337, &counter, 1))) {
+        printf("wloop()\n");
+        counter++;
+        printf("Message sent: %d\n", counter);
+    }
+    led1 = !led1;
+}
+
+int main()
+{
+    printf("main()\n");
+    ticker.attach(&send, 1);
+    CANMessage msg;
+    while (1) {
+        printf("loop()\n");
+        if (can2.read(msg)) {
+            printf("Message received: %d\n", msg.data[0]);
+            led2 = !led2;
+        }
+        ThisThread::sleep_for(200);
+    }
+}
+
+#endif

APIs_Drivers/CAN_ex_1/mbed_app.json

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+{
+    "config": {
+        "CAN1_RD": {
+            "help": "CAN1 rd pin",
+            "value": "NC"
+        },
+        "CAN1_TD": {
+            "help": "CAN1 td pin",
+            "value": "NC"
+        },
+        "CAN2_RD": {
+            "help": "CAN2 rd pin",
+            "value": "NC"
+        },
+        "CAN2_TD": {
+            "help": "CAN2 td pin",
+            "value": "NC"
+        }
+ 
+    },
+    "target_overrides": {
+        "*": {
+            "CAN1_RD": "p9",
+            "CAN1_TD": "p10",
+            "CAN2_RD": "p30",
+            "CAN2_TD": "p29"
+        }
+    }
+}