[ONME-2736] Macro Def. of 6LP-ND Security Mode

* Application has been using MBED_MESH_API_6LOWPAN_ND_SECURITY_MODE as the macro to define Secuity mode.
* The fall back mechanism, in case of absence of neo or yotta macro definition, was setting the macro to
  be yotta format which was not used at all in the application.
* The bug was fixed by changing YOTTA_CFG_MBED_MESH_API_6LOWPAN_ND_SECURITY_MODE to
  MBED_MESH_API_6LOWPAN_ND_SECURITY_MODE in the fall back mechanism.

Author: Seppo Takalo

features/net/FEATURE_NANOSTACK/mbed-mesh-api/source/include/static_config.h

@@ -73,7 +73,7 @@ extern "C" {
 #elif defined MBED_CONF_MBED_MESH_API_6LOWPAN_ND_SECURITY_MODE
 #define MBED_MESH_API_6LOWPAN_ND_SECURITY_MODE MBED_CONF_MBED_MESH_API_6LOWPAN_ND_SECURITY_MODE
 #else
-#define YOTTA_CFG_MBED_MESH_API_6LOWPAN_ND_SECURITY_MODE NONE
+#define MBED_MESH_API_6LOWPAN_ND_SECURITY_MODE NONE
 #endif
 
 #ifdef YOTTA_CFG_MBED_MESH_API_6LOWPAN_ND_PSK_KEY_ID