Corrected json and added check to ensure that needed json options are set

Author: Mika Leppänen

features/nanostack/mbed-mesh-api/mbed_lib.json

@@ -155,7 +155,7 @@
             "value": 0
         },
         "certificate-header": {
-            "help": "Certificate header",
+            "help": "File name of the certificate header file (used on include directive)",
             "value": null
         },
         "root-certificate": {

features/nanostack/mbed-mesh-api/source/wisun_tasklet.c

@@ -29,6 +29,10 @@
 #include "sw_mac.h"
 #include "ws_management_api.h" //ws_management_node_init
 #ifdef MBED_CONF_MBED_MESH_API_CERTIFICATE_HEADER
+#if !defined(MBED_CONF_MBED_MESH_API_ROOT_CERTIFICATE) || !defined(MBED_CONF_MBED_MESH_API_OWN_CERTIFICATE) || \
+    !defined(MBED_CONF_MBED_MESH_API_OWN_CERTIFICATE_KEY)
+#error Invalid Wi-SUN certificate configuration
+#endif
 #include MBED_CONF_MBED_MESH_API_CERTIFICATE_HEADER
 #endif