Update EmacTestNetworkStack

-Uncomment get_mac_address
-Document set_mac_address

Author: Arto Kinnunen

TESTS/network/emac/emac_TestNetworkStack.h

@@ -84,8 +84,17 @@ class EmacTestNetworkStack : public OnboardNetworkStack, private mbed::NonCopyab
          *
          * @return              MAC address as "V:W:X:Y:Z"
          */
-        //virtual char *get_mac_address(char *buf, nsapi_size_t buflen);
+        virtual char *get_mac_address(char *buf, nsapi_size_t buflen);
 
+        /** Set MAC address on the network interface
+         *
+         *  @param          mac_addr Buffer containing the MAC address in hexadecimal format.
+         *  @param          addr_len Length of provided buffer in bytes (6 or 8)
+         *  @retval         NSAPI_ERROR_OK on success
+         *  @retval         NSAPI_ERROR_UNSUPPORTED if this feature is not supported
+         *  @retval         NSAPI_ERROR_PARAMETER if address is not valid
+         *  @retval         NSAPI_ERROR_BUSY if address can't be set.
+         */
         virtual nsapi_error_t set_mac_address(uint8_t *mac_addr, nsapi_size_t addr_len);
 
         /** Copies IP address of the network interface to user supplied buffer