NRF52840_DK: Fix baremetal linker error

Compile in the inclusion of cryptocell310 only if the library is included
in the build

Author: hugueskamba

features/cryptocell/FEATURE_CRYPTOCELL310/mbed_lib.json

@@ -0,0 +1,6 @@
+{
+    "name": "cryptocell310",
+    "config": {
+        "present": 1
+  }
+}

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/objects.h

@@ -54,7 +54,7 @@ extern "C" {
 
 #include "nrf_uart.h"
 
-#if defined(FEATURE_CRYPTOCELL310)
+#if MBED_CONF_CRYPTOCELL310_PRESENT
 #include "objects_cryptocell.h"
 #else
 struct trng_s {