Fix header files used in esp8266

"ns_types.h" results in build failure when networking feature is ignored with
.mbedignored. ESP8266 component should be enabled only when NSAPI is present.

components/wifi/esp8266-driver/ESP8266/ESP8266.cpp

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#ifdef MBED_CONF_NSAPI_PRESENT
 #include <string.h>
 #include <stdint.h>
 #include <stdlib.h>
@@ -1082,3 +1083,4 @@ nsapi_connection_status_t ESP8266::connection_status() const
 {
     return _conn_status;
 }
+#endif

components/wifi/esp8266-driver/ESP8266/ESP8266.h

@@ -17,6 +17,7 @@
 #ifndef ESP8266_H
 #define ESP8266_H
 
+#ifdef MBED_CONF_NSAPI_PRESENT
 #include <stdint.h>
 
 #include "drivers/UARTSerial.h"
@@ -454,5 +455,5 @@ private:
     nsapi_connection_status_t _conn_status;
     mbed::Callback<void()> _conn_stat_cb; // ESP8266Interface registered
 };
-
+#endif
 #endif

components/wifi/esp8266-driver/ESP8266Interface.cpp

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+#ifdef MBED_CONF_NSAPI_PRESENT
+
 #include <string.h>
 #include <stdint.h>
 
@@ -673,3 +675,4 @@ void ESP8266Interface::proc_oob_evnt()
         _esp.bg_process_oob(ESP8266_RECV_TIMEOUT, true);
     }
 }
+#endif

components/wifi/esp8266-driver/ESP8266Interface.h

@@ -17,6 +17,7 @@
 #ifndef ESP8266_INTERFACE_H
 #define ESP8266_INTERFACE_H
 
+#ifdef MBED_CONF_NSAPI_PRESENT
 #include "ESP8266/ESP8266.h"
 #include "events/EventQueue.h"
 #include "events/mbed_shared_queues.h"
@@ -359,5 +360,5 @@ private:
     void proc_oob_evnt();
     void _oob2global_event_queue();
 };
-
+#endif
 #endif