Generate DeviceKey Root of Trust

Since ARMmbed/mbed-os#12385:

    0e7a53c DeviceKey Root of Trust generation refactored.

the Root of Trust is not automatically generated anymore. We need to
generate or inject one explicitly.

This commit also improves the readability of the existing code for the
injection of Root of Trust.

Fixes #71

Author: LDong-Arm

main.cpp

@@ -56,15 +56,27 @@ int main()
     //If TRNG is not available it is a must to inject the ROT before the first call to derive key method.
     printf("\n--- No TRNG support for this device. injecting ROT. ---\n");
     ret = inject_rot_key();
-    if (DEVICEKEY_SUCCESS != ret && DEVICEKEY_ALREADY_EXIST != ret) {
-        printf("\n--- Error, injection of ROT key has failed with status %d ---\n", ret);
+    if (DEVICEKEY_ALREADY_EXIST == ret) {
+        printf("\n--- ROT Key already exists in the persistent memory. ---\n", ret);
+    else if (DEVICEKEY_SUCCESS == ret) {
+        printf("\n--- ROT Key injected and stored in persistent memory. ---\n", ret);
+    } else {
+        printf("--- Error, injection of RoT key failed with error code %d ---\n", ret);
         return -1;
     }
 
-    if ( DEVICEKEY_ALREADY_EXIST == ret ) {
+#else
+
+    // The ROT must be present before the first call to derive key method.
+    printf("\n--- Generating ROT. ---\n");
+    ret = devkey.generate_root_of_trust();
+    if (DEVICEKEY_ALREADY_EXIST == ret) {
         printf("\n--- ROT Key already exists in the persistent memory. ---\n", ret);
+    else if (DEVICEKEY_SUCCESS == ret) {
+        printf("\n--- ROT Key generated and stored in persistent memory. ---\n", ret);
     } else {
-        printf("\n--- ROT Key injected and stored in persistent memory. ---\n", ret);
+        printf("--- Error, generation of RoT key failed with error code %d ---\n", ret);
+        return -1;
     }
 
 #endif